Opera recently released the Opera 10.5, the fastest browser on earth. If you are someone who lives out of a USB stick, you do not need to download and install the Windows version. Previously mentioned Portable Opera, have released a new standalone version of Opera 10.5 which you can run from a USB drive.
If you have already been using an earlier version of Opera Portable, you can also download an updater, which will update your current install to Opera 10.5.
The portable version can also be used to try out Opera 10.5 without having to actually install it on your system. Either ways, I highly recommend Opera 10.5 to users as it is definitely the fastest browser available today.