There's a Minecraft server running at minecraft.exultant.us. Feel free to check it out, and email hash@exultant.us if you require permissions.

There's a small community of folks who build on the server, and we've got muliple worlds (both creative freebuild and survival modes, as well as nether and skylands environments) and a variety of mods (namely WorldEdit, WorldGuard, and CraftBook; all via the Bukkit mod platform).

Stuff built on this server is for keeps: backups are kept in a git repository here, which you're free to clone whenever you want if you feel like moving your creations on this server to a single-player world or any other server. Uptime, however, is not (completely) guaranteed: while the machine running this server is always-on, I am a dev for CraftBook, and this server started as my testing grounds. Therefore it can and will reboot frequently and sometimes behave incredibly erratically if I happen to be hacking that day.