Fall or Spring planted cover crops are especially easy to add to your rotation because they make maximum use of Winter and Spring rains and reach their peak before field or garden space is needed for warm weather crops. Or plant in late Spring, and till in in the Summer. If you have time and space, or are rotating crops, it's a great way to put your soil in rehab.
Most clovers are low growing, attract beneficials such as bees (if allowed to bloom), are great for erosion control, and are good competitors against weeds. Will come back in subsequent years if allowed to go to seed.