Say I'm on OSX and I want to modify some .n64 ROMS, simple stuff like replacing textures and soundfiles. What are my options?
I would probably do a search at romhacking.net. I'm an OS X user myself, but you might want to use a VM for that kind of thing, Windows has a lot more ROM editing utilities available for it.
Tile Molester
N64 texture tools
There's a Mac Version of N64 Texture Explorer, though I don't have a blue's clue how to use it.