Found this really handy yesterday. It's a c binary that wraps your script and checks a lockfile before running it.
e.g. to run 'myscript' if it's not currently running:
lockrun --lockfile=/tmp/mylockfile -- myscript
The cool thing is that it uses flock to implement the lockfile, so the lock is automatically free'd when the process dies. (hence the reason for being a C binary rather than a shell script).
Lockrun is ultra handy if you need to run a command every minute, but don't want them backing up if something causes one to take longer than a minute. Also I guess you could use it to keep processes up - i.e. lockrun exits if the process is already running, but if not it starts a new one.