I think 'Lock My Mac' (http://www.freemacware.com/lock-my-mac) is excellent, simple, and convenient.

Thanks, developer(s).

I hope the next version allows us users to preset a screenshot or screensaver, whereby one tap on the space bar conjures up a LogIn window waiting 60 seconds for the LogIn password or fade again - while recording a QuickTime movie of the person trying to LogIn . . .
Those little QuickTime movies to be automatically destroyed after 2x 24 hours, in accordance with the laws on privacy. So no harm done if no attempts are made.

Now THAT would make for a killer app! Security like a bank!