I found a great guide to time lapse photography that covers all the details before and after this Instructable. They mention the use of an "intervalometer" (a new word to me) which can cost $60 to $150 dollars. That's way too expensive for the casual photographer I think. So I made my own!
Step 1: Planning, Schematic, Materials
Here's what this circuit will do. When you turn it on, the camera will get a false signal and take a picture. Then the big capacitor will slowly fill with charge while the green LED is on. How slowly is chosen by the variable resistor. When the cap discharges, the 555 outputs a signal to the transistor which connects the shutter control to ground. The red LED lights up and the camera takes a picture
There are also buttons to use the box as a remote shutter release when the circuit is off.
As you can see you need some stuff. Here's a list:
Resistors: 100k, 470k, 33k, and one 1M variable
Capacitors: 220µF, 0.1µF
LED's: Green, Red
Transistors: NPN-type switching
555 IC timer chip
IC holder socket (to switch out a bad chip)
Protoboard and then later a Radioshack IC PC Board
For Pentax DSLRs, Canon Rebel's, and maybe some others:
Cell phone headset with 3/32" 3 conductor plug (make sure it works, but go cheap)
Soldering and wire stripping tools and such
IC PC Board: $2.49
Handsfree headset kit: ~$5
Other bits and bobs: ~$10
So it all can be had for under $20 and some effort.