If you're an ASP.NET developer - this tool should save you many hours of debugging and configuration testing. A LOT. It's a browser helper object for IE that shows you lots of interesting stuff about your ASP.NET page.

 

As you can see from the screenshots, I went along and added in some additional tools such as the ability to hide trace information from the page (useful if that interferes with your page layout), and view it in a separate window, as well as the ability to see view state in a decoded form. The latter is especially useful for control developers to analyze their view state and control state usage. I already spotted a couple of things we could potentially pack better in GridView."

 

(via Dino)