In November 2010, Secure World Foundation (SWF) published their X-37B Orbital Test Vehicle Fact Sheet (PDF) that is a conjecture on the purpose of the two Boeing built X-37B OTV (Orbital Test Vehicles) currently operated by U.S. Air Force. It also gives a background on vehicles, their known technical characteristics, and describes official objectives they're supposed to serve by quoting the then “X-37 Orbital Test Vehicle,” U.S. Air Force Fact Sheet:
Official objectives of the X-37B program include "space
experimentation, risk reduction and concept of operations development
for reusable space vehicle technologies."
The U.S. Air Force page linked since changed in contents and URL address, but the new one isn't any more forthcoming, I'd say it's even less so. So all we have is speculation on its possible uses, based on its equally speculated performance characteristics. Educated guessing galore, here's some of the additional uses that the SWF paper suggested, including their estimated feasibility:
- X-37B as an on-orbit sensor platform and test bed (Feasibility: high)
- X-37B as a deployment platform for ORS satellites (Feasibility: medium)
- X-37B as an on-orbit repair vehicle (Feasibility: low)
- X-37B as an on-orbit inspection or ASAT platform (Feasibility: low)
- X-37B as a Conventional Prompt Global Strike (CPGS) weapon or delivery system (Feasibility: zero)
Edit to add: In April 2015, the first speculated possible use in the list was finally confirmed with the DoD news release that X-37B will start testing Hall thruster technology aboard the vehicle that is currently scheduled for launch from Cape Canaveral on May 20, 2015 atop Atlas 5 rocket.
This does increase future possibility of the second and fourth items in the list too, to use X-37B as ORS satellite deployment platform since Hall thrusters, if successfully tested, could greatly reduce their weight and increase maneuvering capability, which could also enable various ASAT (Anti-Satellite) options. Same goes for X-37B vehicles themselves, as speculated by Foxtrot Alpha:
This technology could also help in the development of killer satlets,
parasites and other foreign satellite manipulation technologies as it
would allow these small satellites to pack more maneuvering capability
while also possessing a finer degree of control.
Then there is the unmanned X-37B itself, which is a clear candidate
for such a technology as it is basically a long endurance space truck.
On its last mission it spent 675 days in orbit. With this new thruster
technology, its endurance could be enhanced even longer.