Press Applications -> Add/Remove... and look under the Popularity column. There is a star rating for each package, which is supposed to give some indication of how popular it is, i.e. how many people have installed it. This could be used if you were trying to decide between two similar packages I suppose, although I doubt that it would be of much actual use. Software search results are apparently ranked (in part) by popularity, and some of the data may be used to 'improve the support for popular applications'.
You can contribute to these statistics if you like. Ubuntu doesn't collect information on which packages you have installed by default, so you have to enable the data collection feature yourself. Click System -> Administration -> Software Sources and click on the Statistics tab. Tick Submit statistical information and an anonymous report on how often you use your apps will be sent every week.