Xobni came to mind ... they weren't so very much at the mercy of Microsoft.
One of Xobni's biggest barriers to entry is the difficulty of developing for Outlook. However, that difficulty doesn't exist for Microsoft itself, so Xobni's always run the risk of MS simply packaging all their features into Outlook itself:
One of Xobni's biggest barriers to entry is the difficulty of developing for Outlook. However, that difficulty doesn't exist for Microsoft itself, so Xobni's always run the risk of MS simply packaging all their features into Outlook itself:
http://www.xobni.com/blog/2009/11/18/xobni-and-the-outlook-s...