Quick three from me. All premium stuff though, no freeware.
CSSEdit - A bit niche, but I love this app so much, probably my favourite on OS X. Saves me a lot of time.
Transmit - FTP software.
XTorrent - Only Torrent app on OS X I've been able to tolerate. I've not liked Azureus since I replaced it with uTorrent on my PC, horrible clunky thing. Others haven't worked particularly well or have been too basic. Search thing is rather nifty.
Oh and second the Adium choice. Went through all the IM apps a couple of weekends ago and I found it to be the best one by far.