Apple has announced that their public beta for Mac OS X 10.10 will go live today. It's interesting news for Apple since they haven't done anything like this for a long time (if ever). I'm interested to see how this experiment progresses.
Microsoft has done this occasionally with a public beta of their Release Candidates for Windows. I took advantage of that to try out Windows 7 and experiment with it.
I'm a registered iOS developer so have had access to Yosemite for a few weeks and am very pleased with it so far. I had never tried a Mac OS X pre-release version before so can't really compare it but it feels like a great leap forward over Mavericks. Much more polished than I might've expected and much less buggy than the iOS betas are at the moment. It also feels much faster to me than using 10.9.
I'm really looking forward to the final release of this.