irssi (terminal client)
komanda (webview)
Linkinus (native)
Slate (webview)
Colloquy (native, not very sexy)
xchat aqua/azure (repo, deprecated aqua style)
snak (deprecated aqua style)
More here http://simple.wikipedia.org/wiki/Comparison_of_Internet_Relay_Chat_clients#Operating_system_support
Lot of them not listed because "too old" (ugly, unmaintained)