Windows: The extremely popular instant messaging application Trillian just released a new beta, adding direct replies to emails and tweets via notification popups, instant URL previews (you won't get duped by rickrolls or NSFW links) and more Wikipedia data for quick lookups.
Apart from those solid feature improvements, the new beta release has better Windows Live support, a great new history feature for browsing through sent and received images, improved skinning and all-around faster setup. If you're already a Trillian fan and don't mind a few rough edges, kicking the tyres on this beta release seems like a no-brainer.
Trillian 4.2 Beta is a free download for Windows. (Just be sure to skip the Ask.com toolbar installation.)