avlock -- a system level lock screen replacement for android
i'd always liked android's pattern unlock, but it had one fatal flaw: you could discover someones password by simply looking at the smudges on their screen. also, nodes in the pattern could only be used once, so patterns had a max length. avlock was created to address these problems, and was eventually integrated into cyanogenmod
avlock was different than other lock screen replacements because it was developed at the operating-system level, not as a standard application. this means it can't be bypassed with basic trickery.
you can find the relevant xda thread here
. and here's a youtube video
showing the functionality.