David A. Wheeler's paper "Why Open Source Software / Free Software (OSS/FS, FLOSS, or FOSS)? Look at the Numbers!" provides a large collection of quantitative data that shows that using FOSS "is a reasonable or even superior approach to using their proprietary competition". The paper examines market share, reliability, performance, scalability, security, and total cost of ownership.
Complete Article: http://www.dwheeler.com/oss_fs_why.html