I want to start a "rate my"-style site. The rated objects are mostly texts. I want it to be rather simple. Features I need:

• object rating (thumb up, thumb down)
• object comments
• object tags
• related object presentation based on tags
• user authentication and management
• private message system
• sanity checks for text inputs (i.e. prevention of code injections)
• cache
• open source
• runs on GNU/Linux

I would gladly take something that is tailored for my scenario but a generic framework would be fine too. I simply don't want to write stuff like user authentication that is been written a million times and risking security flaws.

Programming language is irrelevant but python/php preferred.