Some author did this research and here is the result:

1. Sun buys MySQL, $1 billion, 2008
   Sun now has their hands on the world’s most widely used open source database.
2. Red Hat buys Cygnus Solutions, $675 million, 1999
   Red Hat started the open source acquisition race early when they bought Cygnus Solutions, providers of open source software support.
3. Citrix buys XenSource, $500 million, 2007
   Considering how hot virtualization is right now, we can see why Citrix bought XenSource, the company behind the Xen virtualization software.
4. Yahoo buys Zimbra, $350 million, 2007
   Yahoo already have their own email services, and with Zimbra they got an integrated email, messaging and collaboration software.
5. Red Hat buys JBoss, $350 million, 2006
   Red Hat strengthened their SOA offerings by buying the JBoss Java application server.
6. Novell buys SUSE, $210 million, 2003
   Novell got their own Linux distribution by buying SUSE.
7. Nokia buys Trolltech, $153 million, 2008
   Trolltech is the company behind the Qt GUI framework which is used by the popular Linux desktop environment KDE.

Source: http://royal.pingdom.com/?p=245