I just finished this plugin which gives you a preview when you hover on the previous/next navigation links in a single post page.
Sometimes title is not enough for visitors to determine whether the post is worth reading. Some posts with great titles are worthless, while some great stories are under unromantic titles. With an excerpt, more information is provided.
Neighbor Post Preview
Hope you like it. I only tested it with 2.3.2, so if you're using older versions of WordPress, please give me some feedback :)