This is the overview feature description -- see also new features in each release.
MARS features:
GUI with
point-and-click control and integrated
editor. The three icons shown here denote "run", "single-step",
and "single-step backwards." 
Easy
set/removal of breakpoints using check boxes

Easily editable register and memory values, similar to a speadsheet
Display values in hexadecimal or decimal
Command line mode for instructors to test and evaluate many programs easily. Command-line arguments to specify registers and memory locations to be displayed after the program run, to examine for correct contents. Set up a "batch" to do many programs in succession.
Floating point registers, coprocessor1 and coprocessor0
Variable-speed execution

"Tool" utility for MIPS control of simulated devices (new in 2.1). A tool is a program running on a separate thread with access to MARS data. An assembly program can run in MARS and interact with the tool through memory-mapped IO. Any imaginable pseudo-device can be interfaced to MIPS assembly code, or extended to physical devices or hardware.