I love my small modular rig, despite it's numerous limitations and I can't recommend one strongly enough.
They're not for everyone though.
Do you want to build your modules from scratches (ie buying components, circuit boards etc) or just buying the ready-assembled modules and build your own synth?
I did the second, so I bought mine module by module as I don't have the knowledge to do electronics stuff.
If you want to go fulll diy I can only suggest Elby design [Only registered and activated users can see links. Click here to register]
the panther system looks supercomplete!
Or maybe the doepfer diy synth board
[Only registered and activated users can see links. Click here to register]
really really cheap and complete