Casper Electronics
The website got hacked in 2017 and rebuilt from scratch, but old versions containing the diy-related stuff have been archived and can be found in the wayback machine.
Bookmarks with tag “circuit bending”:
Experimentalists Anonymous DIY Archives
Experimentalists Anonymous has been an online resource and information repository for people interested in building their own audio circuits since 2003.
The board seems gone, yet the archive remains.
WarrantyVoid (Weltenschule TableHooters)
A pretty old website with a collection of all kinds of information on thoroughly inspected toy keyboards and often times their modifications.