Pidgin was pretty awesome, though its underlying library was so riddled with vulnerabilities…

I greatly enjoyed how with XMPP’s in-band registration you could do things like give each contact a unique XMPP address, only to be used with them. You could have unlimited (well, I never hit a limit) accounts, across a load of servers, proxies per account, unique OTR keys per account, etc.

I don’t think the UI has really been surpassed either, tbh. I really wish someone would do a modern rewrite of Pidgin in some memory safe language.