Enterprise Microsharing Matrix: Yammer and 14 Rivals Compared - Mashable
http://mashable.com/2008/09/25/enterprise-microsharing-matrix/

Laura Fitton presents a side-by-side comparison of 15 enterprise microsharing applications.

A comparison of 15 internal "enterprise 2.0" tools similar to Twitter

the first side-by-side comparison of 15 enterprise microsharing applications:

Twitter (Twitter)’s “water cooler” collaboration, support and problem-solving qualities have led to much discussion about a similar application for internal “Enterprise 2.0″ collaboration and communication. Yammer’s success touched off a spate of announcements about similar applications, and Jeremiah Owyang and I have been tracking the growing list.