Present.ly is a micro-update communications tool for your company. Give your employees the ability to instantly communicate their current status, ask questions, post media, and more.

Twitter revolutionized public communication by creating organic networks for one-to-many updates. Present.ly brings this revolution to your business with tools and features custom tailored for a work environment.

• A big appeal for corporate IT - you can deploy Present.ly internally, ie, run it entirely on your own network, behind your firewall. Very different to Yammer, which only runs outside your firewall on Yammer's servers (in  the cloud, to quote the tech phrase du jour).
• Present.ly has a sliding scale of account types, ranging from free (the one I've signed up for) for up to 5 users, to $99 a month for 100 users, to a negotiable rate for unlimited users.
• Depending on which plan you sign up for, you can get service features such as file sharing,  SMS notifications (although I didn't check whether this is US only), use from your mobile device, and secure  SSL communication.
• Present.ly lets you create different types of groups of people - useful in a workgroup environment where you might want to inter-act with different colleagues on different projects. It looks similar to Yammer's member feature but maybe it has deeper functionality.
• It already comes with a Twitter-compatible  API which would give corporate software developers a great opportunity to explore ways of connecting Present.ly with other applications within the enterprise.
• You can sign up for Present.ly with a  webmail email address - unlike Yammer, you don't have to have a corporate-domain address.