Favorite app of Mountain Lion (OS X 10.8)
I just installed Mountain Lion on my Macbook Pro! Most new things aren't groundbreaking, such as the new Notifications Center. It's really just Apple's version of Growl.
What I do like is the new Messages application. It's a big update from the legacy iChat app, and really makes chatting much cleaner and easier. Google Talk, or "Jabber", is of course available, and now there's video chat feature built-in. One slightly odd thing is the contact/buddy list is hidden in the (+) plus icon, though typing a contact's name in the "To:" field will see if they're on or not. Maybe Apple is being minimalistic, thinking a contact list is too cluttered. I would agree, but on a usability level, I would like to see directly who's online and offline.
Messages also supports AIM, Yahoo!, Bonjour, and of course, iMessages. Unfortunately, I don't have an iPhone to test out the full capabilities of Messages, but the improvement for Google Talk has been great. I will actually be using this instead of Google's own Chrome extension for GTalk.