OpenALPR is an open source library which analyzes images and video streams to recognise license plates in EU and US formats, ideal for setting up your own automatic license plate recognition system.
This is intended for developers, but the package does include a command line tool, some sample files and a readme.txt file to help explain how it works. (There's no installation required, either, so you can try it out without any problems or hassles.)
Essentially this just involves entering commands like these:
alpr -c us samples/us-1.jpg
alpr -c eu samples/eu-clip.mp4
ALPR scans the image (or every video frame) for license plates, and displays any likely plate characters that it finds, along with a confidence rating.
If you like the idea, but prefer something a little easier to use, OpenALPR's website offers commercial products where your IP camera can be automatically monitored, license plates recorded, and the full history available at any time via a web dashboard ($50/ month per camera).
OpenALPR did a great job for us, recognising most of the license plates we tried without difficulty. The package only has the core libraries and a command line tool, though - it'll take some work and technical knowledge to do anything more than scan a few sample files.