The twin Pioneer spacecraft are storied objects. They have been drifting through space for a long time, almost 30 years. They have added greatly to our knowledge of the solar system. They have also vexed the engineers that built them.
The Pioneer probes are slowing faster than they should. In space, there are very few particles to run into, so drag is essentially nonexistent. The only thing that can accelerate the spacecraft is radiation, usually from thrusters. However, Pioneer has been slowing at an almost imperceptible but still measurable rate, even when there was no apparent source of energy to effect such a change.
This anomaly understandably confused the engineers. Some physicists went so far as to assume that the composition of the solar system must be different way out there. A few even supposed there could be a hidden flaw in General Relativity. Now, it seems like the answer is much more prosaic: heat. The radioisotope cores in the probes generate heat to power the on board systems. This heat, although it radiates very little energy, could account for the slight decrease in velocity.
This is a likely explanation, but a little bland. I was personally hoping for alien intervention. For that, we may have to wait for Voyager One to clear the solar system.