Tumblweed is a tiny Adobe Air-based desktop Tumblr app for Mac and PC.
Tumble exists somewhere between blogging and Twitter, encouraging short messages like Twitter, but also supporting so called rich media like images and video. Tumblr describes itself as a scrapbook for your thoughts, and Tumblweed has been developed with this in mind.
You can do all the creative parts of Tumblr in Tumblweed. Enter text, photos, quotes, links, chat, audio and video. These are in the apps only menu, along the top. They are marked with the first letter of their function, which is fine when you know, but not if you don't, and there is no explanation if you hover over the buttons.
Clicking any of the menu buttons opens a small window - with photos you can simply drag them into the box, add a caption and post. Video requires an embed code, and so must be hosted elsewhere. It's all very simple and works perfectly well.
The problem with Tumblweed is that it does a lot less than than the web app - you don't get a feed of your friend's posts and so can't take part in any conversation. If you want to edit a post, tumbleweed will open you browser anyway, which slightly negates the purpose of having a desktop app altogether.
Despite these criticisms, if all you want to do is post stuff on Tumblr, there are no problems with Tumblweed, and it will no doubt get more features in the near future.