WeBWorK, WebAssign, CAPA, Blackboard, Prometheus, WebCT

WeBWorK and CAPA, and to some extent WebAssign use a "programming" or algorithmic paradigm for developing problems. More flexible -- higher learning curve. In particular WeBWorK allows one to program the answer checking as well as the delivery of each problem.
Others use a "database" paradigm. Fill in the parameters in one of several problem types.
WeBWorK and CAPA have facility for TeX based hard copy output. I don't think the others do.
Things to consider

Extensibility
Ease of use -- particularly for students, but also for instructors
Availability -- especially for students. WeBWorK won't work with a class that does not have easy access to the internet.