Theres good one Bugzilla.
Its free and it is definatelly robust and is web based.
I have been using it for over a year now. And it works - fantastic
Only problem is the installation. http://mozilla.org/projects/bugzilla/
They offer a freeware version that is identical in functionality to the one that costs. The only difference is the freeware does not use a database. It stores every thing as objects.
There is a site, www.defect-tracking-forum.com that lists lots of products, some consultants, and has user feedback mechanisms for the rest of us. The forum is not busy, but there is a requirements generator that will create a thorough requirements spreadsheet. It worked for me.