These are the instructions for my popular design of a laser turret from Portal. It's all Lego; you won't need any other materials to make this nice display piece.

Step 1: Parts Needed

Here are the parts you'll need. You may have some, most, or all of these in your own collection; if not, you can get them from Bricklink.com (or Lego's Pick-A-Brick , but they tend to be much more expensive, if somewhat easier). Part names may be different between Bricklink and Pick-A-Brick.

No. Color             LDraw no.     BL no.          Part name
--------------------------------------------------------------------------------------------------
4    Black              48729.dat     48729b        Bar 1.5L with Clip
2    Dark-Gray      44728.dat       44728          Bracket 1 x 2 - 2 x 2
2    Dark-Gray      4733.dat     4733             Brick 1 x 1 with Studs on Four Sides (color doesn't matter)
2    White              4733.dat       4733             Brick 1 x 1 with Studs on Four Sides
2    Black              47905.dat     47905           Brick 1 x 1 with Studs on Two Opposite Sides (can be Dark-Gray)
2    Black              4593.dat         4593              Hinge Control Stick
3    Black              3820.dat         983                Minifig Hand
1    Black    4497.dat         4497              Minifig Spear
2    Dark-Gray    30162.dat    30162            Minifig Tool Binoculars Town
2    Black              4006.dat         4006              Minifig Tool Spanner/Screwdriver
2    White          3024.dat         3024              Plate 1 x 1
8    Dark-Gray      4073.dat         4073              Plate 1 x 1 Round (color doesn't matter)
1    Trans-Red 4073.dat         4073              Plate 1 x 1 Round
2    Black               4085c.dat    4085           Plate 1 x 1 with Clip Vertical Type 3
2    Dark-Gray       4085c.dat 4085              Plate 1 x 1 with Clip Vertical Type 3 (type 4 is even better)
2    White               3023.dat    3023            Plate 1 x 2
2    Black               3794.dat        3794            Plate 1 x 2 with 1 Stud
4    White 3710.dat        3710              Plate 1 x 4
4    White       3460.dat        3460              Plate 1 x 8
2    White 3021.dat        3021              Plate 2 x 3
4    White               50746.dat    54200           Slope Brick 31 1 x 1 x 2/3
2    White       3048.dat    3048              Slope Brick 45 1 x 2 Triple
22 White       50950.dat     50950           Slope Brick Curved 3 x 1
5    White       61678.dat    61678           Slope Brick Curved 4 x 1
4    Black 4599.dat         4599              Tap 1 x 1
2    Black           41677.dat    41677            Technic Beam 2 x 0.5 Liftarm
2    White       6541.dat    6541               Technic Brick 1 x 1 with Hole
2    Light-Gray       4274.dat        4274               Technic Pin 1/2
1    Black               6558.dat 6558               Technic Pin Long with Friction and Slot
1    Light-Gray      4697b.dat       4696               Technic Pneumatic T Piece - Type II
4    White              3069b.dat       3069               Tile 1 x 2 with Groove

An Excel sheet of the parts with links and more information is attached.

Parts list.xlsx
Download

Step 2: Step 1

Onto the build! I'll add hints down here for the Lego-illiterate. ;)

Step 3: Step 2

Yes, the tiles are supposed to swing back and forth loosely here. They will be secured later.

Step 4: Step 3

Unlike the previous step's bricks, these have studs (the knobby things) on all four vertical sides -- they are often called "travis bricks". This step ties the whole assembly thus far into one piece.

Step 5: Step 4

Step 6: Step 5

Step 7: Step 6

None of the non-white pieces' colors here matter much; they won't be very visible.

Step 8: Step 7

Notice that you do the same thing on the other side.

Step 9: Step 8

Step 10: Step 9

Step 11: Step 10

Step 12: Step 11

This connection may be a little loose; using newer clips or gently squeezing the clips before attaching the brackets can help.

Step 13: Step 12

The 1x1 slopes are called "cheese slopes", one of my favorite parts.

Step 14: Step 13

Step 15: Step 14

This one can be a bit tricky; the bases of the sticks, when put in together, should stick tight in the red plate.

Step 16: Step 15

Fit the previous assembly in the 1/2-plate gap that goes around the body. Try to fit the balls of the lever handles in the gap in the middle; this may take several tries. Once they're in, gently squeeze the body to slightly shrink the gap, to hold the eye in place.

Step 17: Step 16

Did you get the eye in? Great!

Step 18: Step 17

Step 19: Step 18

The tap's top can move around a bit within the slope, making for some nice angles.

Step 20: Step 19

Depending on the parts you have, you may have to pull the hands out of your minifigs' arms. It's okay, they won't mind.

Step 21: Step 20

Step 22: You're Done!

Once you've adjusted the legs to your liking, congratulations! You now have your very own Portal turret. I recommend using it only as a display trinket; it won't hold up to much play.

Step 23: Credits

These instructions were built in LDraw/MLCad, then screencapped in LDView and edited in Photoshop Elements.

Step 24: ...But What About the Laser?

Oh, right. You're wondering how I achieved the laser in my photograph? Don't worry, your turret won't actually kill you.

Materials needed for laser effect:
Laser light (warning: don't shine in anyone's eyes, etc.)
Misting humidifier or something else that creates a mist or fog

Step 1: Shine laser into turret's eye (it's okay, he can't be blinded)

Step 2: Fog up the area with the humidifier to make the laser visible

Step 3: Take picture

Here's my setup:

http://www.flickr.com/photos/ltdemartinet/4653056529/in/photostream/

Introduction: Lego Portal Turret