I'm sure there are other on/offline blogging applications but I've recently taken up using the Windows Live Writer application. It's a WISIWYG editor that enables you to draft posts, save them locally, and when connected to the Internet, post to your blog. I also like the style-import feature. This allows one to suck in the theme of your blog so even when editing locally, you know what the post will look like when posted. Finally, it supports most platforms. I use it on both TypePad and Blogger (and soon on WordPress).
My team from The Freestyle Entrepreneur constantly complains about the TypePad editor. Fine, we're switching to Live Writer! (I'm migrating off TypePad as soon as I get our WordPress site running anyway. ;-)