Here are a few links I have found that may be of interest. If you know of any more that you think should be here, please let me know.
Chris Strellis' FPGA Synth
Chris Strellis' Additive FPGA synth (based on the work of Samuel Nobs and Daniel Engeler).
Emulation of the infamous SID chip, using an external filter.
Xilinx Spartan 3E starter kit
A great starting point for building an FPGA based synth
Build your own FPGA starter kit. Details/schematics and gerbers for a Spartan2 dveleopment board (200k gate)
Burch electronic designs
a great looking board with plenty of extras you can buy to simply plug in, and a good price.
a good range of boards and fpga/cpld types.
Superb tool suite, with lots of features and add-ons, and it's free!
VHDL cookbook online
A great place to start if you know nothing about VHDL
Other usefull links
This 'PDF' contains hyper links to lots Xilinx manuals, a usefull starting place.
Paul Schreibers FPGA 'bits'
Paul's collection of usefull FPGA bits and peices