Microsoft Security Essentials public beta now available to download
You'll need to sign in with your Connect account or register for a new one to gain access to the download - for now. There will no doubt be hundreds of happy seeders sharing the Beta on torrent trackers later today.
Microsoft had originally slated only 75,000 spots for beta testers, though they have since announced that the number will be increased to accomodate demand. Interest will likely be high considering the generally positive reviews SE has generated since we first got our hands on it.
We've got a big gallery of screenshots of SE in action, including it detecting the trojan that I *ahem* intentionally left on my machine so I could test the program fully. I've been very happy so far with SE's level of protection and low impact on system performance.