As fond as I am of WLW, it does pain me to praise it. It's a Microsoft Live product, and there's no OS X version on the horizon. It's a bit much to load VMWare Fusion and XP only to run WLW, but I'm tempted.
There's nothing comparable for OS X. Ecto showed promise once but every time I've tried it I've been disappointed. In particular it made a mess of Blogger posts; WLW handles Blogger quirks and bugs as well as anything can.
Update 5/20/08: The WLW Firefox add-in is not compatible with FF 3! Wow. Wish I'd thought about that before I upgraded. I'd have held off on my XP machines.