Flock is interesting. It's a "web 2.0" application built around the firefox/mozilla rendering engine. Open source. Tied to a variety of blogging services, photo sharing, etc. It bugs me that they're hiding their business model, I assume it's somehow tied to selling integration into other services.
There are versions for OS X and XP, including built-in blogging tools that cover Blogger. I'm going to try it on OS X for Blogger work. It has lots of memory leaks and performance bugs (like Firefox) though, so I'm mostly intereted in the version that will come out with the Firefox 2.0 engine.
(PS. Blogger is agan in crisis mode - posts failing, etc. Wow. Blogger is a living advertisement for the downside of web apps.)