What Are Your Favorite OpenSource Projects/ Programs/Services

There are a lot of open source projects out there that can be very helpful to IT Departments. There are several Ive run across that I didnt even know I needed until I found them, and theyve made my job much easier. Theyve also earned me alot of brownie points (my boss loves free stuff!)

The problem is…. there are ALOT of them.

I thought Id start this thread so you all could post your favorites. After all, the more people that get involved with a project, the faster it matures.

**Edit

For reference please note whether its available for windows or just linux