We use a GDrive spreadsheet, with a new sheet for each week. It's nice because it allows you to set up the times in whatever length of blocks work best for your situation.
In order to allow people to do larger "projects" we don't allow people to sign up more than 3-4 weeks in advance (we don't have sheets available more than a month out) unless they set something up with the Tech Coordinator. In my experience, you need to do this or else people love to block off the same time all year (ie: "every Thursday from 2-2:30 is my computer time) and it becomes impossible to find time for more extensive projects.
We have quite a few people who prefer to sign out the portable (cart) lab instead of bringing their class to the stationary lab. The trouble is that they end up checking it out for quite a few hours at a time rather than getting it when needed and returning it as soon as they're done. Carts are nice, but they tend to serve far fewer students, I think.