Is there a free tool that I can use to test a silverlight application?
I know this is not html, but there must be something, right?
The only thing I know is Silverlight Spy . However, this is not free.
I found this page during google-ing for "WPF firebug like tool". Then I found Snoop . It works very well and is open source. As already mentioned, this is WPF, not a Silverlight-oriented tool, but since both are closely related and link to this page when searching for a WPF-oriented tool, I post this answer here.
Fiddler2 @ http://www.fiddler2.com/
JetBrains made DotPeek . It is currently free (I hope it stays that way, unlike .Net Reflector).
It also converts Silverlight DLLs back to readable source code! I tried this on several SL projects and it works like a charm.
* Note: at the time of this writing, there are still a couple of errors: string concatenations and event handlers generate incorrect code.