It is really easy nowadays to extend your desktops and laptops with more monitors.

For desktops, you have a choice of adding new graphics cards that support more than one display output or you can add USB graphics card that connect to external USB 2.0 ports. Using USB 2.0 graphics cards has an additional advantage that it does not require you to open your desktop machine (which may void your warranties).

For laptops, the USB 2.0 graphics card option exists. Additionally, laptops support PCMCIA interface as well and you get PCMCIA-based graphics cards too.

Here is a list of vendors and links to their graphics cards for you to choose from:

USB 2.0 External Video Cards

A range of USB 2.0 External Video Cards to choose from. The Compact USB VGA Multi Monitor Video Adapter from StarTech claims to support Windows CE 5.0 in addition to the usual Windows clients like Windows XP and Windows Vista. Support for Windows CE 5.0 means that you can use it with your mobile/handheld Windows devices to view the content from your mobile phones on your monitor. Another one, USB 2.0 To VGA Display Adapter, again from StarTech seems to be designed to support higher resolutions (1600x1200 and 1920x1200) and allows you to connect upto 5 additional displays.

USB 2.0 External DVI Cards
Again a wide range of USB 2.0 External DVI Cards to choose from for connecting your digital monitors. Additionally, the Sabrent USB 2.0 to DVI/VGA/HDMI Adapter allows you to use high-definition HDMI interface and allows you to link upto 6 displays!

Video Splitters
Matrox makes 2-way and 3-way video splitter boxes to allow 2 or 3 displays to be connected to the system. The way it provides this capability is to tell Windows that there is only one monitor but with a dimension of 2x or 3x the normal monitor width.