Still doing the basic paper trail of a calendar on each of the computer lab walls but I update the web based every morning. Here is one example: http://www.kenilworth38.org/lab2signout.html ... We have such few labs to get signed out that most online tools are too expensive for us.
We have a shared space on the network server for teachers. Each school saves a spreadsheet on the shared drive that teachers can access to reserve the labs ahead of time.