What would your list of top 5 Open Source project be?  That is a hard problem!  What if you could ask someone at Microsoft?  Well in this interview (http://port25.technet.com/archive/2007/01/29/ossl-roundtable-answers-your-questions-part-1-of.aspx) with the Director of the Open Source Labs at Microsoft, they do list their top 5!  Here are mine:

Linux

Database (MySql, PostGresql)

Apache

Open Office

Firefox