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 modifications around (compatible with Version 2.0 and available at Releases section at SRB2 Message Board):
SRB2Cineblast - A source code mod created by Cinefast which features the option to add bots in Co-Op and Match among several minor and in-progress features.
SRB2Morphed - A source code mod created by Morph which tries to enhance the object customization capabilities by adding new SOC actions.
XSRB2 - A source code mod created by Inuyasha which features an in-progress Single Player rotation, a new gametype called "Shards", as well as a miniature golf zone.
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.
Botanic Serenity - A level pack created by Digiku which consists of the highly detailed Botanic Serenity Zone, which is the first custom level made for NiGHTS Mode, as well as several unlockables.
Tortured Planet - A level pack created by Fawfulfan which consists of nine single player zones, a final boss, three unlockable maps, and a Match rotation spanning ten maps.