The good news is that if you have upgraded to 2.3.340 via OTA (or properly via manual processes), then you can SBF back to 2.3.320 pretty much regardless of how bad you brick your phone. Once you SBF to 2.3.320, then you can "System-Only" SBF to 2.3.340 and then you'll be back to where you are now. Do be careful because that's an annoying process but it's not really all that bad. The first time I had to do it, it took me ~1.5 hours but now I can do that just as quickly as I flash a ROM (now that I've learned how to do it properly and have a machine setup with the proper software on it).
That said, here are the ones I know of (I guarantee I'm missing many, probably some big ones too):
Fission 2.3.1
Rubix 1.9.0
Darkslide v4.1
Tranquility 3.6.0
Ultimate Droid RC2