Feedbooks has a free ebook of The Great Gatsby available to download (the picture link on the left goes to Barnes & Noble's copy, which is $11.99 for the ebook). I've downloaded it and started looking through it, and it seems to be nicely formatted, with none of the typos or formatting issues that seem to be common in some cheap or free classics. I'm not sure whether The Great Gatsby is actually part of the public domain, though, so it's up to you whether you feel right about downloading it.
I actually reread the book a couple months ago, but looking through my posts, it looks like I actually forgot to review it. Whoops! I'll do that soon.