For a limited time you can snag Battle for the Undercity for the amazing price of free. To do so, you just need to copy the code from the newest issue of Dragon Plus.
Mike Mearls and Chris Lindsay discussed this product in an article back in March on the D&D webpage. It's a well balanced adventure in three separate parts. Each part focuses on a different villain group (all of which are interesting and could easily be reoccurring threats).
I've not used the adventure yet, but I really like it and will do so in the future. Good urban encounters can be hard to find.