A probiotic is the only supplement I take on a regular basis. Not to get too far away from the original topic, but the role of intestinal bacteria in fostering a well functioning immune system (as opposed to causing problems) is a field not well researched yet. I just happened to read this article recently, and the concept that some parts of the immune system require intestinal bacteria to function normally really blew me away. These folks also used "germ-free" mice.

The Microbial Health Factor
Just one molecule can make the difference in mediating a healthy immune response. Surprisingly, it comes from bacteria.