If you run multiple computers and monitors in close proximity to each other, Stardock's Multiplicity 2.0 (free and $40 premium editions) is a cheap and effective alternative to a physical KM switch. It allows you to share a mouse and keyboard with multiple computers. See all Software reviews.
Multiplicity is free for non-commercial use with two networked computers. More PCs than that and you'll need the $40 version that provides support for up to 9 PCs and laptops, as well as nice touches such as cut and paste between PCs, centralized audio, and sending keystrokes to all PCs simultaneously. (Pricing is in US Dollars.)
The latter two features are new for version 2.0, as is the AES-256 bit encrypted connection.
Installing Multiplicity is easy, though there's a very subtly placed option for AVG Security Toolbar that's selected by default. Install it if you want it, otherwise, deselect it and follow the prompts.
Eventually the setup will ask you if you want to be a primary or secondary user. Select primary for the PC whose mouse and keyboard you want to use, and secondary for the PCs that you want to control with them. In the latter case, you're given a passcode to enter in on the primary machine to enable remote typing and mouse control.
About the only technical stipulation for using Multiplicity is that the network must be defined on each machine as home or business, not public. Once you've installed Multiplicity on each computer and connected, you drag the secondary computers around a 9-slot grid in the Multiplicity control panel. If a computer is on top of the primary computer, you scroll with the mouse off the top of the primary PC's screen and you see it off on the secondary. Scroll it off the bottom of the secondary screen and you're again using your primary PC.
In my tests, I found the Multiplicity handiest for using my keyboard and mouse to control the laptops I review and network lag was minimal. Beyond keyboard and mouse control, the handiest feature of the pay version of Multiplicity for me was copying files between computers. Alas, while I had no problems cutting and pasting in either direction, for some reason I could only drag items from secondary PCs to the primary PC.