A new way for Spam bot detection. Custom JSP taglib lets you check out delay for postback. The idea is very simple: you can add timestamp to your HTML forms on the rendering stage and use that value for the calculation during the postback processing. Humans cannot submit forms too fast. So if the delay between form rendering (request) and postback is too small than it is very likely that this form was submitted by some software robot. Of course it is heuristic algorithm but it will work. Sure, some artificial delay could be easily added to the bot, but that will cause the delay for the bot itself.
Find out more here: Postback delay taglib