NASA's CubeSat Launch initiative (CSLI) provides opportunities for small satellite payloads to fly on rockets planned for upcoming launches.
Planetary spacecraft originally tended to be large missions. NASA’s New Frontiers and Discovery program enabled a series of lower cost missions that can be compared to personal computers to do the same job. 
The CubeSat concept and program started in 1999 at the Space Systems Development Laboratory of Stanford University. The idea was to meet an educational need to define a meaningful satellite mission that could be developed within a timeframe of a year or two, be of very-low cost, and be of very low mass for reduced launch costs. It turns out that picosatellites offer very promising scenarios to demonstrate new small-scale space technologies and mission concepts.