Microsoft Security Essentials (code name Morro) is now in public beta. I've installed it on a couple of PCs and it seems to be mostly harmless.
For those of you who don't know, MSE is Microsoft's free anti-malware program for Windows. It claims to use a combination of both real-time and scheduled scans to protect against viruses, spyware, pop-ups, Trojans, rootkits et al.
It's replacing their (free) Windows Defender and (paid for) Windows Live OneCare offerings.
It remains to be seen whether it is any good or not - though I don't expect other free antivirus programs like AVG or Avira have much to worry about just yet. No doubt virus writers will be examining it closely for any loop holes as we speak though.