Are you forever rushing back and forth from your Mac but worried that leaving it alone may make it vulnerable to unauthorised access?
If so, try MacLoc, a simple and effective solution to set your mind at ease while you're away from your desk. MacLoc doesn't actually lock the computer itself - it simply activates the Fast User Switching feature built-in to Mac OS X. Before anyone can use your Mac, they have to enter a username and password upon which you'll find everything was as it was when you left your desk.
The handy thing about MacLoc is that you can choose to activate it either from your Dock or the Menu Bar. The Menu Bar option is probably most convenient if your Dock is already full of icons. Whichever method you use, you're best to add MacLoc to your login items so that whenever your Mac starts, MacLoc does too.
MacLoc is a simple and slick app although at the end of the day, it's only using Fast User Switching so it's not exactly bringing anything new to the table.