Adorned with a grid, this mat lets you organize your screws, set them down, and be sure they'll stay where they belong, even in cases of table-bumping or mild earthquakes. On top of that, the surface is dry-erase friendly, so if you feel the need to subdivide sections, or add labels to keep track of which screws are which, you can go right ahead.
While it's magnetized enough to hold your screws, it's not strong enough to really mess up any of your electronics, at least not anything sufficiently modern. The mat's just slightly smaller than your average sheet of paper at 8" x 10" and is available for $15 over at ThinkGeek. If you're sick of losing nuts and getting screwed (or losing screws and going nuts) it might be worth your while. More here.