I'm rather new to the homebrew scene as well, but I suppose I could help.
1) Is there a more decent SNES emulator out there? SnemulDS works fine for some games but is absolutely horrible for others.Not that I have seen, I've tried several SNES emulators, but all of them were shit on the games I played.
3) What other cool emulators are there out there?I'm not entirely sure, but I think I remembered seeing a n64 one. I could be completely wrong though. I also saw something that emulated Doom, and Quake etc.
4) Can I run GBA roms from my CycloDS? I read somewhere that people are saying I need a slot 2 device, but I'm not too sure on how all that works.I have not found out about this yet, I want to know this also.
5) What other COOL SHIT can I put on my DS? This extends to anything.If you're into reading eBooks on the go, download something like DSReader. I use it myself, it's pretty great. As far as I know there's also a comic book reader, but I haven't tried it yet. Oh, and if you like Portal, download Still Alive DS. That shits great.
Here's a couple of links to lists of programs:
http://en.wikipedia.org/wiki/List_of_Nintendo_DS_homebrew There's some great stuff on here. Media players and whatnot, also programs like Photoshop etc. There's also a VoIP homebrew app that I want to try, but I haven't yet.
http://www.dsfanboy.com/category/homebrew/ I've only recently been visiting this, but it's pretty great. Updates all the time with new news of homebrew apps and games. (Btw, check out the Shaving Derby game, it's hilarious as fuck)