FBReader is open source and very nice. It's got a teeny little status line which I like. On my Nexus One, the trackball will scroll the text one line at a time, which is handy. Its library search works well and within a few seconds of installing it, I had downloaded a dozen classics--War and Peace, Moby Dick, Adventures of Sherlock Holmes, etc. Just really easy, and all free.
Kindle is slick and has long-touch dictionary pop-up. I've bought a couple of books with it so far and it's very painless and easy to spend money. But the Kindle library also has free stuff. I had trouble re-syncing when I did a factory reset, but the last time I did it, it sync'd my books again with no trouble.
Google Books seems similar to FBReader and lacks a dictionary feature, so far. It synchronizes nicely with the desktop. It lets you switch to "scanned" mode so you can see the original printed page from which the text was obtained (I'm talking about a free scanned e-book now).
From what everyone says, Aldiko is great. But I'm awash in readers now and would like to just use one great one. But not sure yet which is the best; they're all a little different and strong in different areas.