For Windows I always use Hexchat, which seems to be the only decent open source client for Windows. If anyone would like to jump in and point out others I don't know about, by all means.
I really like Quassel. It's main model is client-server (you install the server component somewhere and it'll stay online, bouncer-like, and you can connect to it from multiple clients), but you can use it in a standalone mode too. The main client runs Linux/Windows/Mac, there's one for Android too.
I just use Thunderbird... It "just works" for email IRC and RSS - and it's very consistent btwn platforms. Maybe I'm missing out on some secret IRC features?
(Heck, it's what I use on Linux too)