NASA has recently announced a number of potential Discovery missions which are being considered for approval. The Discovery class is the least expensive type of mission that NASA operate with a cost cap of $450 million. A number of proposals have been developed by various science teams. Three have been selected for further funding to work up the options. Following that, one more may be selected as a 'real' mission. The three missions being considered are a sample return mission to an asteroid, a probe to study Venus' atmosphere in more detail, and a mission to map the gravity of the moon and provide more details of its internal structure.
Also being considered are three proposals to use NASA existing probes for a new purpose. This concept sounds really interesting. Examples are reusing Deep Impact, which sent an impactor smashing into comet Tempel 1 last July, to visit another asteroid or even use its camera to look for extrasolar planets. Another example is using the Stardust probe to fly by comet Tempel 1 to look at the crater caused by the Deep Impact mission. These inventive uses for existing probes sound like a lot of common sense, lets hope they get approve.
Lots more details on these proposals over on the excellent Planetary Society Blog.