Libera Chat is currently the biggest player in IRC.

Awesome IRC

IRC clients

Web:

• KiwiIRC
• Gamja IRC

Installeable Applications:

• Halloy (Windows, MacOS, Linux)
• Konversation (Linux only - KDE)
• KVIrc (Windows, MacOS)
• Quassel IRC (Windows, MacOS, Linux)

No longer developed (RIP):

• HexChat