UberBukkit lets you run most of b1.7.3 plugins on earlier beta versions. Currently supports b1.0 to b1.7.3.
Report any bugs you find at our Discord server (linked above).
NetherMP is an a1.2.1_01 (a0.2.3) server mod that enables Nether dimension in vanilla multiplayer.
No longer supported!
VersionMirrorer downloads all Java Edition versions from the Mojang servers and checks for the updates. Reupload-proof.
Beware: enormously high CPU usage in v1.x versions, to be fixed in v2.