Since few weeks we have started internally use an interesting microblogging tool named Yammer. It works very similarly to Twitter, which means that you are posting very short posts, focusing on the stuff you are doing or working on. Unlike with Twitter, Yammer is company oriented and your posts will be visible ONLY to the recipients that share your company's email address.
Yammer is free service with interesting business model: everybody can join your company's network (if that person has proper email address), but to gain administrative rights, you have to pay dollar a month per user. I actually like this approach because it gives hope that the service will be around and available next year :-)
Yammer offers browser version (very nicely done, btw), desktop client for both Mac and Windows and (finally) very usable iPhone application. This means that you can follow and communicate with your colleagues even while sitting in the bus or train. You can also post by email and receive posts via email, if you choose so - either as individual emails or as daily summary.
Latest update added option to create groups for really focused communication and managing the channel noise.
So far the uptake and experience within the company was very positive. What I really like is that it allows good alternative and communication middle road between instant messaging (which is too interactive and thus too distracting) and emails (which is too little interactive). It helps create better information flow and keep in touch with people that are out on client's location - which is very important for a professional services organizations like us. It is also extremly important for distributed teams, and because my crystal ball foretells strong likelihood of geographical expansion in very near future, it may become even more useful.
Too bad this service (or Twitter for that matter) did not exist back in 2003 - I could have used it a lot while working in distributed development team spanning 6 timezones.