I have a bunch of Linux VPS servers and run Irssi on one of them. Combined with the GNU Screen program you can access this permanently connected IRC session from wherever you are. All you need is an SSH client, such as Putty, to connect to it.
I have Limechat on Mac & iPhone, which both connect to znc - so I'm "always connected".<p>If someone mentions my name while I'm logged into znc, then znc-notifo pushes a message to notifo (which lets all my devices know.<p>I'm pretty happy with it.
Linkinus for OS X. It is great, fast, clean, looks beautiful. Soon should have node.js scripting support as well.<p><a href="http://www.linkinus.com" rel="nofollow">http://www.linkinus.com</a>
Silverex[1] is what I used to use back on windows. Nowadays, irssi with screen running on a server.<p>[1] <a href="http://www.silverex.org/" rel="nofollow">http://www.silverex.org/</a>
macirssi<p>If push came to shove i would actually probably look up xIRCon for Windows, if i had to use Windows (yes even though xIRCon was already abandonware in 1999, eat that mIRC).