There are many ways to produce artificial gravitational effects, such as a rotating spacecraft, linear acceleration, or harnessing magnetism. Possibly there are other ways to produce gravity that have yet to be discovered.
When we talk about sending people to places beyond Mars, the lack of gravity quickly becomes a problem. It seems that this (along with faster transportation) is one of the biggest obstacles to outer solar system and deep space exploration by humans.
So, what work is currently being done to explore, develop, and test different methods of producing real or artificial gravity?