Now this is cool, Microsoft has added embedded database support to ASP.NET 4 allowing you to create web applications with ASP.NET & SQL Server CE 4. Scott Guthrie (@ScottGu) explores this in more detail:

My Thoughts? This is a fantastic feature for those of us who are inclined to write occasional small personal web apps; we are spared the development overhead of having to develop a full-fledged SQL DB. Now all of you still using MS Access on your web apps, get with the program! One concern I do want to see more information to assuage is concurrency - obviously this solution would be fit for apps that have more than a couple of users but ScottGu's post doesn't mention much about this. Feel free to share your thoughts in the comments!