If you have been curious about multi-user game development you may want to look over my post on MMO Flash Services.
PlayerIO offers a complete package that you can try out completely free until you have something you decide to scale up and monetize. A free SDK includes quite a lot of samples, code and a server you can run on local to test with. You also can set up your game on PlayerIO and share with testers and/or friends. There are links to other tools you will need to work with the server portion of the solution, such as the free Microsoft Visual C# Express IDE.