Opera has tab stacks where you drop tabs on top of each other, which groups them together. Then you can collapse and expand the groups.

I don't use it that much (mostly because there are no shortcuts and command line interface for adding to a stack or expand / collapse), but it is nice sometimes. I might do some research for a blog post, end up with 5 sites that I know I will need to consult when I write it, and just put them in a stack and have it sit there (collapsed, so it doesn't take up much space) until I do write it.

Having said that, I rarely get over 50 (usually in the morning after clearing out my feeds, or when programming against some API / library, when I open lots of documentation pages).

Once I get to the point where I can't see the favicons anymore, I start looking for something to close.