This one is good and thorough – by Colin Purrington, Department of Biology, Swarthmore College. Short excerpt from the beginning:
Why a poster is usually better than a talk
Although you could communicate all of the above via a 15-minute talk at the same meeting, presenting a poster allows you to more personally interact with the people who are interested in your research, and can reach people who might not be in your specific field of research. Posters are more efficient than a talk because they can be viewed even while you are off napping, and especially desirable if you are terrible at giving talks. And once you have produced a poster, you can easily take it to other conferences. If you don’t like to travel far, or are broke, many college and university science departments sponsor poster sessions that welcome students from nearby institutions. For all of the above, session organizers typically have a “Best Poster Prize Committee,” which awards fame and often cold hard cash to deserving posters. And when you’re ready to retire your poster from active duty, you can hang it in your dorm room to impress your friends, or display it in your departmental hallway so that faculty can show off your hard work to visitors for years to come. You can also submit your final product to ePosters.net, which promises to keep a PDF version of your poster in perpetuity (for free) and allows people to send you comments about your poster.