I'm a heavy user of Tumblr. Is there an Ubuntu desktop client that would let me see Tumblr posts from my dashboard and tag which ones I'm following?
There are two clients that I know of; Grumblr and OpenTumblr. I don't think you can follow people from within them, but they work for just about everything related to posting.
If you want the full features of Tumblr, you can even create a desktop application from within Chromium or using Mozilla's Chromeless technology.