Tip: Switch palace switches normally disappear if you replay the level. However, if you insert them as direct Map16 and use the hex-edit at $00EEB2 in the ROM map, you can replay the level without a problem.
Asar is a Super Nintendo assembler based on xkas v0.06. It's the assembler used primarily on SMW Central.
Asar's got a freespace finder, will ask for filenames if double clicked, can use parentheses and proper priority in math, can assemble SPC-700 and SuperFX codes, and has many, many other neat little features. The full list of features can be found in the manual.
New features in Asar 1.70/1.71:
The fullsa1rom mapper now supports freespace searching.
incbin ranges can now use math as an alternative to unprefixed hex. To use this, surround the math with parentheses. For example, incbin file:(4+2)-($F+$10).
Fixed quite a few crashes.
Sped up patching by ~2x.
sfxrom should now properly handle freespace.
For a full list of changes, please check the changelog.
Source code can be found on GitHub, where you can also contribute using pull requests.