I thought these charts, which I found via WPTavern on CMS distribution among the most visited sites online interesting.
When expanded to the top 100,000 and top 1,000,000, WordPress usage goes up to 56.95% and 69.82%
These stats are collected every week I think so the numbers will probably be different next week, though I wouldn't expect them to change significantly week over week.
Mouse over the pies in the chart to see the %. Also if you're not sure what CMS to use, the chart will point you to a number of popular ones.