Favorite Software for Computer Inventory?

Let's say I have about 50 windows computers to manage - Desktops and Laptops for end users. I'd like software to track things like installed software versions, mac addresses, hardware specs, etc. What software do you like for that sort of thing? The more auto-discovery the better. Free and/or Open Source is good but not a requirement. What are some features in this sort of software that you couldn't live without?