I use the CCS64 built-in monitor. I love that monitor. It's not very user-friendly to non-programmers, but now that I'm used to it, I use it for cheating, debugging, SID-ripping, programming... (And you should see some of the stuff I've been
programming!)
I've never tried Action Replay but I probably should take a look at it one of these days. There's been a lot of talk about it, I know that.
Ah, Spindizzy. Truly a classic. The version I got was one that had the cheats built in, though. It had one of those "Press Y for infinite time" screens right after the intro. That's the easiest way to cheat, really: Letting the crackers and demo-coders do the work for you! Hah hah hah!