Windows Live Writer
Windows Live Writer (WLW) is hands down the best, if you are on Windows. One of the things I love about it is its plugin architecture and how many people have written useful plugins for WLW.
(source: mikeschinkel.com)
Qumana
I searched far and wide for an equivalent on the Mac and didn’t find a match but came close: Qumana:
(source: mikeschinkel.com)
Qumana still can’t touch WLW because it doesn’t strip garbage HTML (like from a Word .doc) like WLW does and it doesn’t have a plugin architecture, but I keep hoping… 🙂