Schrödinger is a pre-packaged Java Tomcat installation that provides integrated debugging, logging and other niceties (such as Dreamweaver integration) on Mac OS X:
Besides helping you figure out exactly what went wrong in your JSP code, it provides full HTTP request dumps in a nice, GUI-friendly way.
Enough to get me away from vi? Hmmm... Dunno. But it might be useful to somebody. It even runs on Linux and Windows...