There are a lot more options than you think - here are 13 different options for users of Windows PCs. Don't see your choice on the list? Share it with us in the comments!
Portable Firefox (current) [download] and Portable Minefield [download]
John T. Haller's Portable Apps project is always right on top of the latest Firefox updates, so it's the place to go if you need the Fox in your pocket. The most recent unstable branch is provided as well, in case you want to test it without installing.
PocketFlock v2 [download]
The kicked-up Firefox remix for social networking types is also available in a portable version, thanks to enthusiast Dave Ferrite.
KMeleon is somewhat like a lightweight Firefox. It's a very solid little browser which supports custom skins and is highly customizable. Several modified Firefox add-ons are even available from K-Meleon Extensions Central.
TOR Browser Bundle [download]
The fine fellows behind the TOR project also provide a portable, all-in-one privacy-enhanced Firefox bundle which includes TOR, Polipo, Vidalia, and the Torbutton add-on.
note: these browsers aren't portable in the truest sense of the word. They do,however, provide a good way to use a beefed-up version of IE with your personalizations wherever you can plug in your USB drive (and IE is installed, of course).
Avant Browser [download]
Avant offers mouse gestures, an RSS reader, an ad blocker AND Flash filter, form filler, skin support, and online profile storage. The split-window browsing feature is also pretty slick.
This little baby adds tons of missing features to IE: ad blocker, download manager, URL aliases, form fill, and much, much more. Tons of customization features let you tweak GreenBrowser any which way you choose
Maxthon Portable [download]
Maxthon has a lot more fans and great number of features you probably don't know about. Like GreenBrowser, it adds ad blocking, a download manager, and form fill. Want more? How about mouse gestures, skins, screen capture, online favorites storage...
Opera is an excellent web browser, and it's certainly got a devoted fanbase. Pop this on your flash drive if you like integrated mouse gestures, speed dial, torrent downloading, a built-in mail client, and all the other Opera goodness.
Want some added anonymity whilst you surf? Like the Firefox pack, this zip contains a ready-to-roll portable Opera browser with Tor and Polipo to give you a little extra privacy.
Google Chrome Portable [download]
Carsten Knoblauch's project lets you take Chrome (currently v. 22.214.171.124) with you on your flash drive. He updates frequently and the page's download link should always hook you up with the latest version.
Packed as a .7z archive, Midori is small, lightweight, extensible, and totally portable. It's got an ad-blocker, mouse gestures, and Userscript support. Midori just made the jump to Windows recently, and it's a fairly young browser - so expect the occasional glitch.
Would it really be a surprise if I told you that this one was built using the QT framework and Webkit? No, I didn't think so. Out-of-the-box QTWeb sports an ad-blocker, private browsing mode, and quick access to all privacy-related features.
SRWare Iron [download]
Take Chromium and strip out the bits that feed usage info to Google (like the unique identifier and suggestions features) and you've got Iron.