There are a couple of programs that might work.
Plain text/HTML editors:
* For OS X, there's TextWrangler, Taco HTML edit or Smultron, all free text editors with features for typing raw HTML.
* For Windows, Crimson Editor, Notepad++, HTML Kit and First Page fullfill the same function (the last two are web-development specific)
* Nvu is a cross-platform wysiwyg html editor. You could compose in Nvu then copy the source into the html view, though the html produced by Nvu may not all be allowed by WordPress.com
Blog editors - There are also a number of applications designed specifically for blogging, allowing you to create, edit and manage posts offline.
* Windows: w.Bloggar and Blogjet work with WordPress and WordPress.com powered blogs. Both are free.
* Cross platform: Bleezer and Qumana are free, cross-platform (Win and OS X, at least) desktop blogging clients
* Firefox extensons: Deepest sender and the Performancing for Firefox extension both work with WordPress.com, although I've had some problems with line breaks in DS
* Mac. I'm not aware of any free, OS X-native blog editors (I think that both Bleezer and Qumana use Java, which can be kind of slow), but there are two relatively inexpensive commercial desktop editors: MarsEdit and ecto. ecto is wysiwyg, though it is occasionally buggy (a new version is in the works).
None of these applications are a perfect solution, and an autosave feature would be invaluable, but hopefully they at least give you some ideas. Just Google the names, I was too lazy to type in the links.