Slimlock is a simple screen locker that borrows the interface of SLiM. It automatically reads your SLiM configuration to lock your screen with the same theme you use to log in.
It's lightweight, but not too much. You could try gnome-screensaver, but that would be silly if you don't have the rest of gnome installed. There are other lighter screen lockers, for instance slock and xscreensaver. But xscreensaver is too gaudy for me and slock is too minimal. Here are screenshots comparing slock and slimlock:
Yes, that's right. Slock is just a black screen that goes away if you enter the password correctly. It's kind of cool but it makes me uncomfortable. I could picture someone coming along and shutting down my computer because the screen is locked but they think it's frozen.
I also thought it would be neat to make it match the login screen I use when starting my computer, kind of like Windows... Thus slimlock was born. Enjoy!
Color scheme stolen from the Mustang Color Scheme for vim.