Microsoft Communicator. If you're in a big company that already uses MS Exchange for emails then it's a pretty easy integration. Probably not a great solution for small companies due to the cost.
The O365 plans that include Lync aren't really that expensive, surprisingly enough. It's not a reason to switch, but for companies already used to Exchange, it's probably a viable option.
Office365 makes the cost pretty low. And Lync has got to be one of the better MS products. It blows other PBX/presence solutions out of the water.
Probably the main reason it hasn't completely dominated is because MS is taking a weird partner-oriented approach to VoIP with it. So getting setup with some numbers and outbound calling is tons more work than it should be.
We predominantly use our feet. Unless someone's snowed in, sick, or subcontracted (and not local) we just walk on over to their desk and talk. Or, hold an ad-hoc meeting. When someone's physically absent from the office, we have no standard policy (though Skype is common).
Skype is good too but be careful if you use it for IM. I frequently receive messages hours after the other person sent them, even if we're both online. This is starting to make me not want to rely on it other than for voice/video.
We use Google+ for loads of internal communication. It's easy to restrict posts to our organization via Google Apps.
While I've not seen any use to adopt G+ in a public way, it's been a phenomenal internal comms tool for async sharing, discussion and debate that isn't critical enough to go on an everyone@ email list.
Internal XMPP. Poor ejabberd isn't handling our growth in traffic very well and gets oomkilled and/or swaps itself to death with depressing regularity, so we're trying prosody (over the protests of the internal-IRC faction).
VoIP, email and very occasionally an internal messenger. The idea is not interrupting others too lightly, since we already have scheduled meetings. If something needs dealing with urgently, then these measures kick in.
Engineers use primarily IRC and collaborating with non-engineers is in a Hangout, although we'll use Hangouts too when just need to over something in detail over a short amount of time.
I'll second this. We have a team in New York and a team in Paris, and flowdock has absolutely been the best hub for seeing what's happening across teams, and communicating with each other
I used to be on a team which used SameTime. Although I disliked using Lotus Notes for email SameTime was a joy, especially the ability to take and send screenshots from the chat window.
We also figured out you can add arbitrary gifs as "emoticons" with built-in keyboard shortcuts and everything. Great fun ensued sending terrible reddit gifs to communicate :-)
