Windows only: Microsoft today released a new, limited beta for their increasingly popular anti-malware application, Microsoft Security Essentials. The beta introduces a new protection engine claiming better performance and protection, a network inspection system designed to protect against network exploits, token Windows Firewall integration (doesn't sound all that impressive) and enhanced web protection for anyone who happens to be using Internet Explorer.
Available here after signing in with a Windows Live account, Microsoft Security Essentials beta is available to a limited number of users on a first-come, first-serve basis, so if you're interested, we'd suggest you get it while it's hot. Other than that, it's a free download for Windows only, available in 32- and 64-bit flavours.
Note: For whatever reason, neither the 32- or 64-bit download would complete (they both stalled a couple megabytes short of the complete download before the server was reset). That means I wasn't able to test it, but because it's a limited beta, I wanted to note its availability as soon as possible. Your mileage may vary.