In an effort to keep new PC sales from slowing down while people wait for the release of Windows 8, Microsoft is offering an upgrade to Windows 8 Pro for just $14.99 for new Windows 7 PCs purchased between June 2, 2012 and January 31, 2013. This is aimed at end users though, so there is a limit of five upgrades per person. More details can be found here.
Additionally, Microsoft is attempting to drive the adoption rate of Windows 8 (and hopefully migrate folks still using XP) by offering an upgrade to Windows 8 Professional for any computer currently running Windows XP, Vista, or 7 for $39.99. See here for details.