This page gives my recommendations on the major Linux distributions for newcomers to Linux based on price & requirements. Note that unless you have a really good reason to, i don't recommend the commercial distributions, because for most purposes they don't offer any better functionality than their free counterparts, and come with their own limitations.
Purpose
Free distributions
Commercial distributions
Desktop
  1. Ubuntu (or Debian for IT professionals)
  2. OpenSUSE
  1. Ubuntu
  2. SUSE Linux Enterprise
Server
  1. Debian
  2. Ubuntu
  3. OpenSUSE
  4. CentOS
  1. Ubuntu
  2. SUSE Linux Enterprise (but OpenSUSE preferred)
  3. Red Hat Enterprise Linux (but CentOS preferred)
Gateway
  1. Debian with Shorewall as the firewall
  2. Endian Firewall Community Edition
Smoothwall