Firefox!

Firefox! Yay! I'd expect just about every windows user to get it, except maybe those who use Opera. Maybe. Forked from the mozilla code (which, if I remember correctly, was in turn forked from the netscape code), Firefox is a lightweight, yet powerful browser with plenty of customisation options. You've also got superb tabbed browsing, and it's finally being supported by people doing major sites. Sadly though, my bank still doesn't see it as a supported browser. Bastards.
What customisation options, I hear you scream? Extensions, themes and search integration are the three categories that pop into mind, although I generally find that the default theme and search engine do pretty well for me. Extensions, however, I use a lot. Yarr, time for a list of the extensions worth meantioning that I've got at the moment.

• Adblock Plus
  It blocks ads. What more can I say? Lots, actually. It's very useful for getting rid of image and flash based adverts, as well as blocking bits of javascript and the like. Certainly cleans the web for most.
  
• Browse Images
  Let's you... Browse... Images... Yeah. Self explanatory, up to a point. Most useful when you're browsing galleries. Has functions to save images, open them all in one/separate tabs, and a few other things.
  
• DownThemAll!
  Great little download manager, supports resuming far better than the built-in one, as well as mass-downloading of stuff, multichunk downloads, the works. Get it.
  
• FireFTP
  Nice little FTP client, means I don't need a separate program to do it.
  
• Google Browser Sync
  Essential if ye've got more than one PC. Syncs browser settings, bookmarks and all that between instances of Firefox. And it's Google.
  
• StumbleUpon
  Click stumble, see a site, rate it. Etc.
  
• Stylish
  Lets you add custom CSS snippets for specific (or all) sites. My personal favourite is cleaning up the google custom homepage.
  
• Web Developer
  The web developer toolbar! Very useful on its own to find little problems with sites. Allows you to do stuff like outlining block level elements, labelling classes and IDs and putting the site through a verifier. Very useful for coming up with CSS for Stylish, I find.

All of the above should be findable at the Firefox addons site, or are at most a quick google away.