Hi,
I haven't used a C=128, but here are some things you could use with the 128 in C=64 mode (maybe 128 mode as well, not sure).
If you're into electronic music, you could have a look at the Prophet64 and its related projects:
Prophet64.commssiah.com (I'm definitely getting one of these babies when released)
bigmech.com Prophet64 mods (this stuff is pretty cool to me)
I'm not sure if anyone has mentioned it here, but there are some C=64 drive emulators for a PC, which can allow a C=64 to use files on a hard drive (with some limitations).
Have fun,
Oli