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 1.80/1.81:
Labels are allowed in assert commands
The print has a bin() function. Additionally, all of bin()/dec()/hex() take an optional argument to pad to a specified width
There is an option to emit an address-to-line mapping, which can be used by some emulators to provide source-level debugging.
New functions floor(), ceil(), bank(), datasize(), and a new <: operator.
Support for macros with a variable number of arguments
Support for global labels, which ignore the current namespace
Various internal improvements to make patching significantly faster
For a full list of changes, please check the changelog. For more details about new features, see the manual.
Source code can be found on GitHub, where you can also contribute using issues and pull requests.