Apart from all of the excellent projects here there are a number of places on the web that can be good sources of ideas on how to make things.
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Step 1: Google Books
Google books contains the full set and in full view of the "Popular xxx" run of magazines going back to the early 1930's/1920's.
You can find them by going to Google books.com and click on browse books and magazines.
Select Advanced book search just under the search books box.
Select full view, magazines and enter into the search criteria Popular Science (or Popular mechanics.) as required.
This will give you all the copies of "Popular xxx" since they were first published.
You can if you like further refine your search by adding some criteria into the other search boxes.
Step 2: Project Gutenberg
Project Gutenberg is a vast repository of out of copyright books and has many books on various aspects of engineering and science. Although old, in many cases the information is by and large still relevant and if superseded by modern techniques may still give you some valuable inspiration.
Step 3: Vintage Projects
Vintage projects is a collection of old 1950+/- projects formally printed in various practical magazines of the period. Many are still relevant and valuable.
Step 4: R Q Riley
This is a DIY projects site - They sell full plans for many transport and similiar projects from recumbent bikes to hovercraft.
Although they primarily sell plans there is often enough information in the write up for a capable maker to go ahead themselves.
In the end you can always buy the plans.
Step 5: Steam Engines Etc
Jon Tom is a collection of plans for building model steam engines and similar the plans are PDF files and can be down loaded for free.
For those who know, all of Elmers plans are here with permission of the family.
Step 6: UNIQUEPROJECTS
UNIQUEPROJECTS This is a source of many project plans as PDF files.
The plans are graded and fairly easy to follow.
Many differing projects
Step 7: Yesmag Projects
Yesmag projects is a large collection of fun things to make and do. Largely aimed at kids but then most of us are just kids at heart.
Very good to do with your children.
Step 8: Science Toy Maker
Science toy maker is as the title suggests a collection of links to many other sites that cover the more scientific side of making.
Step 9: TEACH SOAP
TEACH SOAP explains how to make your own soap, body lotion etc. Fun if you like smelly things.
They really do work I have made several items from this site at a fraction of the cost of buying more or less the same thing.
Step 10: Stevenson Projects
Stevenson projects for the boat makers out there have a number of boat related plans for sale they also have a somewhat rare set of plans to build a child's pedal car based on the MG TC styling.
Although this site has a much better build over view of the MG TC car project built from the plans.
Many pictures available that almost make the plans necessary.
Step 11: Land Yacht
Interested in building a Land yacht? Well http://www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/Build-a-Landyacht-Lake-Lefroy-Mini-Yacht/ shows you how - many pictures.
Step 12: Boat and Canoe Building
There are many sites out there that will show you how to build a boat or canoe - Not as complicated as you might think and assuming your not building an ocean going yacht not too long winded either.
Several years ago I built a 14 foot sailing dingy for about £50 over a period of around 4 weeks.
Aerolit boats have one of the most innovative and different build methods. They sell plans but there is a lot of good information on the aite.
Interested in canoes? http://www.duckworksmagazine.com/03/r/projects/frye/skin2.htm shows how.
Small weird sailing dingy
How to make a sail from house wrap
Some good boat building theory
How to stitch and tape up a boat
Step 13: Paper Modelling
Lots of sites out there, some with better instructions than others. As this is very much an eastern hobby many are in Japanese.
http://www.ss42.com/pt-scifiction.html covers science fiction models.
http://www.fiddlersgreen.net/ claims to be the worlds biggest site for paper models.
http://www.yamaha-motor.co.jp/global/entertainment/papercraft/ odd one from Yamaha with many ultra detailed motor bike models.
http://cp.c-ij.com/en/contents/1006/ many models from Cannon the camera people.
http://www.datenform.de/ppeng.html just right for a rebuild. Paper Pixel is a punch card driven display.
http://paperforest.blogspot.com/ is a blog on paper art - lots of inspirational pictures short on how to.
http://www.yeesjob.com/v8engine.htm indescribable level of detail in this paper V8 Engine
http://paperrobots1999.com/home.html Paper robots (non working although with a little ingenuity they could be).
http://shin.co.nr/ more paper robots and models.
Step 14: Fligt Simulators and Motion Platforms
Always wanted to build that realistic flight simulator that allowed you to experience not just the visuals but also the actual motions of flight or driving that NASCAR? http://www.acesim.com/main.html is a good start.
http://mypage.direct.ca/b/bsimpson/byoc~1a.html build a cockpit simulator
http://forums.overclockers.com.au/showthread.php?t=914885 hiding the driving simulator as furniture
http://www.kolthar.com/mysim.htm a racing simulator - Not too sure on the level of detail here.
http://www.playseat.com/en/playseat-f1-race-simulator-launch This costs around £1000 just begging for a rebuild - My instructable is underway.
Step 15: Robots
Building that "Robby the robot" must be everyone, who dabbles in electronics, dream. Lots of sites out there of varying value here are just a few.
http://www.buffingtonfx.com/j1432/animatronics/arvid.html Animatronics primer - fairly good basic information to get you started.
http://www.taomc.com/bits2bots/ lots going on here
http://www.howtoandroid.com/HowToBuildRobotHead.html build an android head for $600 - assuming you wanted to!.
http://sbolt.home.xs4all.nl/edz.htm Dizzy, My favorite Robot - Small simple engineering but still manages to appear aware of it's surroundings with some clever microprocessor work.
http://letsmakerobots.com/ a mixed bag some with enough information to allow you to repeat the ideas.
http://www.solarbotics.net/library.html nothing on Robots would be near complete without the solarbotics site.
http://www.stiquito.com/ an interesting application of shape alloy wire to drive a walking robot.
http://home.comcast.net/~botronics/index.html#picbug cute little robot projects using mainly Picaxe.
http://www.shadowrobot.com/airmuscles/overview.shtml if you have a source of compressed air (a soda bottle will do pumped up with a bike pump) then this is a very easily re-purposed idea from Shadow Muscles.
Step 16: Kites
Step 17: Kinetic Models
I essence Kinetic models move. Many are art forms in their own right.
http://www.woodthatworks.com/ many fantastic and beautiful wooden kinetic models.
http://woodgears.ca/machines.html Matthias Wandel's site is a wonderland of what you can do with wood that no one thought of doing before (well hardly anyone.)
http://www.robosushi.com/ robots or Kinetic art? You decide.
http://www.reubenmargolin.com/waves/ Boy would I like to make this (if only I had the space)
Step 18: Jewellery
http://pineneedlecasting.homestead.com/Main.html a neat way to make an abstract pendent.
http://www.anvilfire.com/iForge/tutor/vi_copper_c1/top_index.htm how to lost wax cast.
http://www.sculpey.com/projects.htm polymer clay projects Sculpty (Fimo in UK)
http://www.ganoksin.com/ information and technique source.
http://www.craftsfaironline.com/Jewelry.html index of jewellery sites.
http://blog.londonjewelleryschool.co.uk/2011/07/17/its-a-wrap-free-wire-wrapping-jewellery-project-from-ljs/ Wire jewellery projects
http://allcrafts.net/jewelry.htm beading and jewellery projects
http://portalgc.knowledgebase.net/display/2/index.aspx?c=12004&cpc=TKvN0Y442nJq41250385h0TtoO3RuDHqcReVI&cid=11047&cat=&catURL=&r=0.926252901554108 Cooksongold one of the best UK suppliers for jewellery materials - Large Knowledge base.
Making Jewellery isn't that hard, It's very creative and much cheaper than buying the same product.
Examples of my work above.