new product: SchmartBoards - easy to solder Surface Mount prototyping break-out boards, even .4mm pitch, QFN and BGA.
Curious Inventor - Kits

Kits (open-source & hackable)

with all sorts of documentation and explanations
by Wim Van Gool (Belgium)
This kit lets you add up to two serial ports to a Linksys WRT54G(L) router. By re-flashing your router, you can run custom code to turn it into a wireless "routerBot" that controls servos, or talks to other electronics via serial (like an iPod or RS-232 port on a computer). Each port can use either TTL or RS-232 levels depending on wiring. Skill level: Intermediate - requires fairly simple surface mount soldering.
by CuriousInventor & help from r.garcia
This converts a PS2 controller into a MIDI controller. The current firmware supports remapping of the buttons to different notes or controllers. It's based on a PIC microchip--full, explained source code and instructions for making modifications are supplied. As a promotional start, there will be several cash prizes (thru paypal) for feature additions. See the rest of the kit page for details. Skill level: beginner / intermediate
by David @ uCHobby.com
This power supply module plugs straight into common bread boards, allowing you to cleanly and easily power your board with a wall wart plug or with wires into screw terminals. It features a variable voltage regulator that can be set to output 3.3 or 5V with a jumper, or any voltage if a potentiometer is added. The input has a rectifier that accepts AC or DC (polarity doesn't matter)--just make sure the input is about 2V greater than the output you want. Skill level: beginner
Loosely based on the Atari Punk Console with some custom tweaks and an LFO front end, this box makes a ridiculous amount of different sounds (mp3 coming soon), all from three 555s. Watch the video or download the mp3 for some samples. Comes as a bare-bones kit, an everything-kit with knobs and case, and a fully-assembled and tested version. It's open source and has small area on the pcb for easy circuit bending. Comes in a bare-bones kit, everything-kit, and fully assembled version. Skill level: Beginner / Intermediate
This is a 10 step analog sequencer, which basically steps through 10 different voltage levels that are set by the black knobs. The output control voltage and trigger signals are typically used to control pitch, filters, drum triggers and an endless number of other parameters on other synths. You can get just the pcb or a kit (bare-bones and everything) or a fully assembled and tested version. Skill level: Beginner / Intermediate