WLA DX Assembler
The somewhat 'classic' Multi Platform Cross Assembler Package that often got used for SNES Homebrews. Nowadays many people prefer other solutions because WLA still have unfixed Bugs.
CA65
Not really an pure SNES related solution. But the CC65 compiling suite includes everything needed. Most of the more recent things ive saw (2008+) tended to use CA65.
BASS
Well. Bass, created by Byuu and taken over by ARM9 is a pretty different Assembly compiler. It owns a special workflow, multi platform support and recent patch abilities. Last one makes it the perfect compiler for modding an patching existing games.
The official Documents are still not public. To google for SNES Manual 1.pdf and SNES Manual 2.pdf is illegal. So please dont do so.
Hardware
Everdrive
Since the everdrive, an SD-Flashcard for SNES came out, the world of snes testing became nice and shiny. This is the product with various features.
Samples
Daniel Oaks Dev. Kit
Well. Daniel did a whole dev enviroment for SNES games. In fact, it is a nice start for people that just want to get some code running and dont want to mess around with the details. It uses CC65 whitch is allready an excelent choice. The trouble starts as soon you try to change something in his Matrix-Grid of makefiles.
ARM9's samples
ARM9 Also did his own bunch of samples. He is trying to implement each feature of the snes HW at least once. Very nice!
Krom's samples
Krom did some nice color extending and windowing samples. Check them out