Light-Locker
The light-locker program is a locker that integrates with lightdm to allow session locking. light-locker can be installed via the light-locker package.
Troubleshooting
A common issue with light-locker is a significant delay during wake-up from locking the screen, if it wakes up at all. This may be resolved by having an entry for your graphics driver in /etc/xorg.conf.d/.
An example for a minimal entry for the intel driver would be in /etc/xorg.conf.d/20-intel.conf:
Section "Device" Identifier "Intel Graphics" Driver "Intel" EndSection