Lighter than air is an autonomous flying robot. It utilizes blimp design principles, combining a helium filled envelope with 3-channel motor control allowing it to navigate in 3 dimensions.

The basic components are:
PIC16F876A microcontroller
2 L293D H-bridges for motor control
Linx rxm-315-lc-s for optional RF control
LM386 op amp for sound amplification
SRF04 Ultrasonic Rangefinder for proximity detection
lever switch to detect when sitting on the ground
3.6v 2100 mA lithium polymer battery for motor power
3.6v TL-5104 Tadiran high energy lithium battery for logic power

The tadiran batteries are available Digikey for cheap or Radioshack for expensive and fit in standard AA holders. The lithium polymer batteries can be purchased at your local hobby store (if you are lucky enough to live an an area that still has one!)
This project is still in progress and components/schematic are constantly changing, but here is the basic schematic.
RA0-RA4 pins on microcontroller go to a header for easy sensor swapping, so no sensor hookup is included in the schematic. The current version uses a seperate SRF04 sonar module handled by a PIC16F648A connected to RB0 as an external interrupt.

I programmed the pic in CCS C. The code has been through a number of iterations based on the different sensors I have tried. Current version uses only one lever switch and one capacitive switch. Next version (in progress) will incorporate differential sensing of two capacitive proximity sensors. Here is the C source code directory for the blimp and the transmitter. The blimp uses the rand function found in STDLIB.H

Contact me if you are looking for hex files or any other information on this project, I would be happy to hear about your own endeavours and give you pointers from what I have learned.

Links:

west coast blimps - 1.5mil metallicized nylon for balloon making. They also sell tons of great blimp parts and handmade envelopes
quantum - cool capacitive touch sensors of all stripes
microchip - pics of course
linx - RF communication chips, very easy to use
barebones pcb - cheap PCBs fast
toytronix - small efficient motors, call and ask for the replacement vertical pusher motor from the ballooncraft blimp they sell
Acroname Great robotics site, $25 SRF04 Ultrasonic rangefinder purchased here
digikey - everything else...