I received a Kindle as a gift in December of last year, and have been using it to read public domain books from Google Books since that time. As a tool for genealogical research (well, background research), the Kindle has been helpful, and I love it because it represents a nexus of my inner bibliophile and Scrooge. For the past year I have been using my Kindle to read a variety of books on early California history, as well as the wagon migration to California across the plains, all for free.
One other way to use the Kindle for research purposes which I haven't seen mentioned is Instapaper. At heart, Instapaper is just a simple bookmarking tool, but the benefit of it is that you can export the content of your bookmarks to a Kindle-friendly format, which you can then move to your Kindle.
I have a hard time focusing enough to read long articles online at my computer, so I "instapaper" longer articles to read at my leisure in my prime reading time, which is in bed before I go to sleep. The translation of the web page to the Kindle isn't always perfect, but it is nice to be able to read a web article or blog post--photos, graphs, and all--in my bed, even though I don't have a laptop at the moment!