WAIT! Don't throw out that old laptop or desktop. You can give them a new lease on life. Just take a look at my Instructable.

Step 1: Tools You Will Need

Well as with most instructables you'll need some tools.
I used:
A screwdriver
torx screwdrivers
and a pliers

Step 2: Give It to the Kids

Are you tired of hearing "Daddy/Mommy can I try your laptop?(That's right the $1600 Mac Book)" and you worry and stress over it. Well you can stop worrying! Just take your kid aside and tell them "Joey I have a surprise for you!" Then give them your old laptop (and some games) and watch them shriek with joy while you can enjoy a world free of stress about the kids breaking your laptop!

Step 3: Use As a Cheap Word Processor

Remember that paper you've been wanting to type forever, but whenever you log onto a computer you always get distracted by your email, games, or this website. Well you can boot up your old PowerBook and type up your paper. Then when you're done you can put it on a floppy and then put it on your main computer to print it up. This would work from Mac(apple) to Mac(apple) or from PC to PC.

Step 4: Covert That Trash to Cash!

If none of these ideas appeal to you and you would rather turn you old computer into cash take it to a vintage computer collector. You might get payed lots of cash for a rare old system.

Step 5: Linux to the Rescue!

If your system is old and outdated you might want to consider putting a version of Linux on it.
Damn Small Linux
http://damnsmalllinux.org/
and
Tiny Core Linux (10mb)
http://www.tinycorelinux.com/
are both good operating systems for an old computer with a small hard drive.

UPDATE:
liquidhandwash reccomends PuppyLinux.
Found here: www.puppylinux.org

Step 6: Strip Down

If you've got an old machine that uses obscure parts you can take parts from these laptops and fix it. For example you can take RAM cards, Hard Drives, Processors, cables, power supplies ,and cases from the old machine and use it in a project you could also hold on to the scrap parts and use them to cobble together another machine.

Step 7: Last Resort

If you are really not interested in any of these things please take your computer to a proper recycling plant. Don't just throw it in the dump. This releases toxic chemicals into the environment and in the year 2020 nobody looks forward to drinking a nice clear glass of water laced with carcinogens and heavy metals.

Step 8: Do You Know What You.ve Done!!

Congratulations!
You have successfully kept several ounces of heavy metals and other toxic chemicals out of the landfill and this planet that we call home. When these chemicals are in your machine they are inert(harmless), but when you put them in the weather at a landfill they leech out into the soil around them. SO please do your part to help save the world. Remember it is always better to have computer chemicals in your computer than in the environment.