Don't forget the world famous OpenServo project. (V2 and V3)
This board can replace the board in your existing 'cheap' servos or you can make your own and get many advantages such as motion profiles and logging and far better and more accurate control.
One of its advantages is the ability to communicate with the servos directly using I2C (or via USB using a USB to I2C converter). So you can control many servos with very low overhead.
So no PWM etc…
Can be purchased from Sparkfun (V2 only) http://www.sparkfun.com/products/9014
or RobotFuzz http://robotfuzz.com/OSv3
(out of stock)
Hi, ginge of robotfuzz/openservo here.
The OpenServo communicates via I2C, which the Pi does have one of.
Doing some research, the Pi's I2C bus is 3.3v rated, while the OpenServo is 5v. You can easy get a voltage shifter or do something quick and dirty (http://www.headfuzz.co.uk/node/46
From what I can tell (correct me if I am wrong) the Pi dosn't have a usable i2c kernel driver yet, but once it does you can drive an OpenServo very easily. There are linux based samples on the OpenServo wiki.
oh, and the "out of stock" on Robotfuzz… the Chinese supplier went bust so I am left high and dry for a little while. I am in talks…