Synth.nl has published a nice series on building a PAIA 9700s modular synthesizer:
It was a lot of fun building and it was actually my first experience with a modular at all. And after building this I just wanted to build more. The kit supplied by PAIA is very complete and the instructions in the manual are [...]

The video shows cursor and bar control for 8 daisy-chained Stribe1s, brightness control and full display animation.
About The Stribe kit:

A modular version of the original stribe project.
Each Stribe1 has 1 softpot touchstrip and 2 columns of 64 LEDs.
Right now, you need to already have an Arduino or some other microcontroller interface to use a Stribe1. [...]

Here’ something to be thankful for - laser harps!
Nortec Collective is building a laser harp, from a design made by Stephen Hobley:
Luis Vargas our technician guru is in charge of making this instrument work, the plan is to use it on our live presentations for controlling our synthesizer, Here is a preview, coming soon in our [...]

The construction and installation of the Highly Liquid MIDI2600 MIDI interface board into a fully functional Atari 2600.
Features:

Translates incoming MIDI notes to joystick or keypad control signals for any Atari 2600 console
MIDI-controlled Select & Reset outputs
Powered by game console—separate power supply not required
User-selected MIDI input channel
Power and activity LEDs
Compact size: 83 x 57 mm (3.25 [...]

This is the most musical demo I’ve seen of the cheapster DIY Gakken SX-150 synth. 
On the video, I’m using Ableton Live to send midi data to the processing sketch using a Network Midi connection on the Mac. On windows, you should be able to use Midi Yolke to accomplish the same thing. Then, I make sure that [...]

This is a demo of Auduino, the Arduino synthesizer.
Auduino is a synth DIY project, based on the Arduino board. The audio is coming direct from Arduino PWM output with only a slight external reverb. No other effects were used.
Description:
The Auduino is a sound synthesiser based on the Arduino platform. The prototype uses the Arduino Diecimila, although any modern [...]

This is a demo video of the Whacky Sound Generator, a synth DIY project from Music From Outer space.

This is an accelerated assembly video for the $29.95 Gakken Mini-Theremin kit, via Make Magazine.
Description:
The theremin, invented in 1919 by Russian scientist Leon Theremin, is one of the world’s earliest fully electronic instruments, and is also unique in that it was the first musical instrument designed to be played without being touched. The eerie, other-wordly [...]