If you have an analogue watch and a laser mouse, you can easily keep your computer "active" by setting the mouse on top of the watch, no software or computer tweaking necessary.
Whatever compels you to keep your computer active — watching a video and want the screensaver to stay off, don't want the terminal to lock itself while you're working on some paper notes, etc — this terribly clever hack can help. Simply place your laser mouse on top of an analogue watch. The movement of the second hand will activate the mouse and move the cursor a tiny amount. When we tested the trick the mouse moved a few pixels every 30 seconds or so as the second hand swept around the watch face. Check out the video above to see it in action.
For a software-based solution check out previously reviewed Mouse Jiggler for Windows and Jiggler for Mac.