It’s called rein you can get it here. Mind you, I just switched today after using Whitehart for about ~5 years.
Chromifox looks very good too, but it’s not original. Plus, it doesn’t integrate well with the Aurora GTK engine/theme.
Here is a screenshot of rein in action on my Xmonad + Vimperator setup (with Terminus as the GUI font, of course).
You gotta love those tabs. Finally, a theme that looks clean, simple, and unique.

Fancy Tab Trees?
IMO they’re easier to work with when having >20 tabs
and if you’re on a widescreen monitor (as you should be)
then it makes better use of the width.
Just a thought
Comment by asdf — May 25, 2009 @ 2:15 am
Using vimperator, I have “h” and “l” hotkeyed to previous tab and next tab. Lightning-quick hotkeys like these have made > 20 tab scenarios a non-issue. Indeed, I would hate to use the mouse at all to navigate tabs, especially if there are fewer than 5 of them as using “h” and “l” is so much quicker. Plus, I have my widescreen monitors rotated 90 degrees now (easier for programming), so I do NOT want to use anything to make my viewing area horizontally cramped.
Use vimperator!
Comment by Shinobu — May 25, 2009 @ 4:13 am
hehe.. I _do_ use vimperator
People gasp when I tell them this. But I started using vim
only after I’d liked working with vimperator so much
I guess I can’t live anymore without the hierarchial tree view
(esp. in wikipedia where its really useful to know what exactly
got you to where you are
But a 90* twist on things would be fatal to most such niceties
Comment by asdf — May 26, 2009 @ 4:38 pm
“[Hierarchical tree view is] really useful to know what exactly got you to where you are”
Excellent point. I just realized that my previous comment is half-worthless because I did not realize that hierarchical tab tree view can coexist with vimperator — i.e., I can use all my lightning-quick vimperator hotkeys alongside the tree view, although it is a bit odd now that “h” goes “up” the tree and “l” goes “down” (changing them to “k” and “j” respectively would not work as they are already hotkeyd to vertically scrolling the page up and down). (Yes, I installed tab tree view just to get a feel for it.)
I’m still not totally sold on the idea, though =p (my surfing habits are very efficient and I pretty much never get lost in the sea of tabs on my firefox window — even during those all-day browsing/research sessions).
Comment by Shinobu — May 27, 2009 @ 5:12 am
Well I use ^P and ^N to navigate tabs in a tree.
And hjkl are for the current tab’s navigation.
Comment by quant — June 6, 2009 @ 6:37 pm
I had it set up like that too when I first installed vimperator, until I realized that I almost never needed to scroll horizontally within a tab. So I swapped h & l for ^P and ^N, respectively.
Comment by Shinobu — June 7, 2009 @ 5:18 pm