Mudlet is available for Windows, Mac, and Linux.
MUSHclient is on Windows, most of us use it.
TinTin++ is on several platforms.
BeipMU is on Windows.
We're mainly working on plugins and such for the above, but the game is playable with any MUD client.
Grapevine has a web-based client, it is customized to show status bars for hit points and such.
Mudslinger is another web-based MUD client.
MudPortal is another web-based client, the developer appears to have stopped working on it.