Here are some useful links if you want to get into building gear yourself.

• www.ucapps.de - Thorsten Klose's MIDIbox hompage. Highly recommended if you want to build
  your own SID synth or other excellent machines!
• SmashTV shop: for buying the Midibox parts in USA. Great beginner-friendly PCBs.
• Mike's Elektronik Shop: the same for buying in Germany, but different PCBs.

Other nice projects to check out:

• MIDIBOX FM synth: uses the early 90s Soundblaster chips (OPL3). Very unique machine, with the
  FM sound of early 90s games like Monkey Island. Or like a Yamaha DX-7.
• Shruti-1: great small monophonic synth with optional analogue filters, that can be built in one day.
• Meeblip: another amazing small synth, not unlike the Shruti-1, but with more knobs and a
  different concept.
• Highly Liquid: very good Retrofit MIDI kits, so you can play old Casio (and other) synths via the
  computer, a AKAI MPC etc.

Recommended literature, if you want to get into soldering and circuit bending:

• Nicholas Collins: Handmade Elecronic Music. Great for beginners, no whacky theory, straight up
  into building fun stuff right away.
• Reed Ghazala: Circuit Bending - THE classic from the man who invented the term. Includes many
  very well explained bending walkthroughs for synths and toys.