If any product will push DisplayLink into the mainstream it's this.
For just $115, it allows you to add a second screen to your computer over USB rather than D-SUB or DVI. It's compact, but a little messy - there's no VESA mount.
It doesn't integrate extra buttons into Windows applications and it only supports resolutions of up to 1440 x 900 or 1280 x 1024, where ideally it would support more standard resolutions, up to 1680 x 1050 or 1600 x 1200. But it worked smoothly and, importantly, it's cheap, making it ideal if you need to add a screen to your system on a tight budget.