The whole idea of Software As A Service (SaaS) and espcially Google Apps idea has taken off in 2010. In addition to applications obtained directly from Google, there is a whole community of developers producing apps which are complimentry to Google Apps. These application can be found in the Google Apps Marketplace. What is surprising so far is the catgory of applications that people are choosing to install.
In order of popularity:
The great thing about these applications is they tend to enable collaboration right out of the box, and being available on the cloud they are really good at enabling this collaboraation across remote locations.