One of the top reasons why SRB2 became so popular is because it allows for massive customization; users could make characters, levels, music, graphics, or complete EXE modifications.
Below lists some of the most popular SRB2 MODs around (compatible with Version 2.0 and available at Releases section at SRB2 Message Board):
SRB2MFE - An EXE MOD created by Jason the Echidna which mainly fixes the scaling giving better gravity and instant shrinking on map load.
SRB2Python - An EXE MOD created by Yoshibot which has an embedded Python interpreter in it and SWIG bindings to expose some of SRB2's functions to the Python interpreter allowing for more dynamic and sophisticated scripting.
Acid Missile - A Level Pack created by FuriousFox which has three zones, a final level, emblems to look for, and a few unlockables to tide players over.
SRB2 The Past - A Level Pack created by Glaber which ports maps from previous versions of the game as well as old versions of current maps to 2.0.