26.8% Mountain Lion 10.8
28.0% Lion 10.7
35.1% Snow Leopard 10.6
10.0% Leopard 10.5 and Tiger 10.4
If we assume that half of the macs running Snow Leopard (10.6) are pre-2007 and ineligible for 10.9, we have about 72.3% macs that can upgrade to the free Mavericks, which is great news for Mac App developers.
[1] http://www.zdnet.com/ad-network-os-x-is-becoming-fragmented-...