These are said to happen about twice per century and we are due for one. The last one was in the sixties with the Apollo project to the moon. According to the theory, they have always happened-- but there really aren't that many data points over two hundred years.
If you consider all of the ideas that are percolating these days, such as:
- SpaceX and Elon Musk's proposal to colonize Mars, and his ideas for reusable rockets
- Gingrich talking about Moon bases by the end of the decade
- Commercial space initiatives, ie Virgin Galactic, XCOR, amongst others
- Quicklaunch, which is a large gas gun, which can put materials in Low Earth Orbit (LEO) cheaply
- Stratolaunch, which is an air launched rocket that can reach LEO safely with quick turnaround