Firefox. Especially with the extensions it offers.
A few examples of ones I've got:
AdBlockerPlus - Blocks banner adverts, exceptionally good for getting rid of those Flash ones with the mosquito.
MouseGestures - Allows you to hold down a mouse button and drag a shape, this does something (e.g. for me, left line = go back a page, down then right or right-left-right scribble = close window/tab)
ColorfulTabs - Purely aesthetic, makes entries on the tab bar have individual colours, rather than grey.
Download Statusbar - Adds a bar to the bottom of FF when you're doing a download, meaning you don't have a Download popup, just a visual representation of how far the download is at the bottom (Can be set to change colour depending on current download speed as well)
Link Alert - Shows me what I'm linking to. E.g. a link to a .exe file will cause an extra icon next to my link-clicking cursor showing the ".bat cog" icon. Two squares means new window, etc. V. useful from a security POV.
IETab - Essential if you're planning on using FF only. This little gadget puts an icon on your status bar, in the bottom right, with the FireFox logo on it. Click this icon, it changes to the IE logo and Firefox is now emulating Internet Explorer. This basically means you can run Windows Update and other IE-only sites through firefox (can also be configured to always go into IE-mode for certain websites)
All of these can be found if you go into the Tools menu and click "Add-ons" and then the blue "Get Extensions" link. Use the search box there. I'm pretty sure I've given the names correctly, and a couple of the ones I've said (It's pretty much all the ones I have, thinking about it!) are in the "Featured" list. All rather useful. (There's even one I haven't tried yet which allows the PC to shut down automatically after all downloads are done. Useful for that 1.8Gb Bioshock demo if you want to get to sleep and know the PC'll be off by the time you wake up)