Looking for web based bug/defect tracking software.....

And I'm having a hard time finding one I like. I am interested in what you folks have used for web based bug tracking software...? Please give us all some good links, etc. in my comments section as I'm sure others are interested in this as well.

I'd like something that is written with .NET and is extendable but I'd take just about anything that looks good and works well. I only have about $500-600 dollars to spend so keep that in mind.