Firefox. It's not just about tab browsing and "quick link" bookmarks (which rock, by the way), but also some of the extensions:

- Flashblock: Makes it so any flash that loads in your browser needs to be manually clicked on first. This means that you don't get interrupted by annoying flash banners that just waste bandwidth (and not to mention sometimes play sounds you don't care about).

- AdBlock: You can use a wildcarded expression to block any url from being accessed by firefox. Essentially, any page you go to a lot (facebook or whatever) is as ad-free as you want it to be.

- FireBug: This is more interesting for developers, but this thing shows you all http requests (and timing for them), DOM details, various debug messages, and all kinds of other REALLY nifty stuff.

- GreaseMonkey: For a given site, you can install a custom javascript to modify the page into pretty much anything you want. For example, if I wanted the "++ Quote" button on this site to open the "quick reply" box and manually paste the quote in, I might be able to do it (with a bit of work).