I helped with a previous quadrotor build (Instructable here), and after flying it I decided I wanted to make my own. I had an idea to make a miniature one comprising a single printed circuit board that is both its structural frame and its electronics motherboard. The end result was "4pcb", a 138-gram micro quadrotor. I designed it in EAGLE, soldered it, and tested it over the course of a few months. Here's some video of it flying, after a lot of control tuning (see Step 13) and practice:
More flight video in the final Step!
The idea of making a PCB-based quadrotor isn't unique (see links below for other examples), and 4pcb definitely isn't the smallest (see the Picopter Instructable for a really tiny one). But I think it strikes a good balance between size, cost, buildability, and flyability. It's also one of the only PCB quadrotors with integrated brushless motor drivers, so there's no need to wire up external ESCs. And it runs on XBee digital radios, so there's no RC receiver or servo-style wiring.
Size: 6.50" (165mm) motor center-to-center distance, diagonally
Battery: 3S (11.1V), 370mAh, 20-40C Lithium Polymer
Motors: HXM1400-2000 "hexTronik 5gram Brushless Outrunner 2000kv"
ESCs: Toshiba TB6588FG "3-Phase Full-Wave PWM Driver for Sensorless DC Motors"
Props: 4x2.5 (2), 4x2.5R (2)
Controller: Arduino Pro Mini 328 - 5V/16MHz
IMU: Pololu MinIMU-9
Radio: XBee Series 1
Total Weight: 138g
Additional Payload: <30g
Flight Time @138g: 8min
4pcb is a "low level" quadrotor build, by which I mean that there are very few black box components. The frame, motor control, flight control, radio interface, and ground station UI are all developed from component or sub-module level. Depending on your level of experience and interest, you may want to take a different approach where you buy commercial modules for some parts and DIY others. (I included links to some kits and modules below.) This Instructable includes all the files and information you would need to build one completely from scratch.
There are a few small changes I would make if I did a second version of the board, but overall, I think it could make a good standalone project or, even better, a great starting point for your own modifications! (6pcb hexrotor, anyone?) Here are some resources that you might find useful, whether you are building this particular quadrotor or a different multirotor:
Other Quadrotor Instructables:
quadrotor - Custom frame with Arduino-based controller.
RC Quadrotor Helicopter - Off-the-shelf frame with custom controller.
Picopter - A very tiny custom PCB quadrotor.
Multirotor Frame Kits:
Multirotor Controller Kits:
HobbyKing (based on KK Multicontroller)
Parrot AR.Drone - Very stable iPhone-controlled quad.
General Multirotor Resources:
RCGroups Miniature Quadrotor Thread
DIY Drones Quadcopter Forum
OpenPilot Multirotor Forum
PID Tuning for Multirotor (OpenPilot TV)
Mini Quads (5" props)
TinyCopter (custom build)
BabyCopter (custom build)
Turnigy Integrated PCB Micro-Quad (KIT) (commercial)
Blade mQX (commercial)
Micro Quads (4" props):
4pcb (custom build)
Kawaiicopter (custom build)
Nano Quads (3" props):
Nanocopter (custom build)
Nano quadcopter wii (custom build)
Pico Quads (2" props):
Walkera QR Ladybird (commercial)
CrazyCopter (custom build)
Picopter (custom build)
Chibicopter (custom build)
NC-ONE (custom build)
4pcb and other Flying Things
The Balance Filter - Merging accelerometer and gyro signals.