The next big thing is space probes looks to be very small. "Nanosatellites" -- fully-functional satellite systems measuring a few dozen centimeters in length. As we noted last June, NASA is working on nanosats as sort of micro-UAVs for inspecting the space shuttle and station. Canada's University of Toronto Institute for Aerospace Studies, Space Flight Laboratory (UTIAS/SFL) is working on something a bit more complex, however. CanX -- Canadian Advanced Nanospace eXperiment -- is a series of increasingly-complex, very small satellites, able to work either alone or in "flocks." Multiple nanosats are potentially much less expensive than big solo satellites, and allow for greater flexibility and robustness: the loss of a member of a flock doesn't threaten the whole mission.
The second nanosat in the series, CanX-2, is set to launch in mid-2006; the launch will test a new propulsion system, custom radios, cheap & tiny attitude sensors and actuators, and a commercial GPS receiver. CanX-3 is already being built, with even greater functionality, and CanX-4 and 5 are in design.
Nanosat development would be particularly useful for countries and regions without extensive space programs, as they would allow more collaborative space-based environmental monitoring and research at lower cost and lower risk.