If you spend any significant amount of time working at a desk, the single best way to boost your productivity is to connect a second monitor.
Most laptops have either a VGA or HDMI output, making the addition pretty simple: just connect the LCD, tweak Windows' settings to extend your desktop, and then enjoy the benefits of a double-size workspace. (My preferred arrangement: Outlook "permanently" open on the right, my browser and other stuff on the left.)
Ah, but why not go a step further? Why not extend your desktop even further with a third screen?
Simple: few desktops, and almost no laptops, have a second VGA or HDMI output, meaning there's nowhere to connect that third screen.
Ah, but there are workarounds. If you happen to have an older and/or spare laptop lying around, you can install MaxiVista ($39.95). It leverages your laptops' network connection to extend your primary Windows desktop, meaning there's no physical connection required between the two machines.
Another option: take advantage of a spare USB port, which can drive a third monitor when connected using a USB-to-DVI or USB-to-VGA adapter. You can find these adapters online for around $40-50.
Finally, don't overlook monitors expressly designed for second- or third-screen duty. Right now I'm liking the AOC e1659Fwu (shown above, though mysteriously without its cord), a 16-inch LCD that's actually powered by USB, meaning it requires no separate AC adapter. I wouldn't call it portable, necessarily, but it's definitely easier to maneuver than a traditional monitor. Plus, it works in both portrait and landscape modes.
The e1659Fwu currently sells for $129.99, which strikes me as a pretty decent price--and lots of online resellers carry it for under $100. The only hiccup: the resolution is on the low side at 1,366 by 768.