Helma is noteworthy for a couple of reasons:
Multi-app environment: A single Helma instance is an environment for multiple web apps.
Filesystem-based development: Helma uses the FS to structure web apps.
Convenient services: Helma provides a lot of convenient services, like a cron-scheduler and logging.
Zero-configuration: To run Helma, just dl it and run the start script. Presto. Creating a new app is equally simple.
Advanced HTTP support: Stuff like convenient access to request headers and parameters, and file upload monitoring.
Mature: Helma has been actively developed, used, and refined for years (decades?) in a lot of installations, and there's a sizable community.
Helma also has ORM and a sophisticated templating framework, but I don't know them well.
On the downside, Helma can be rather hard to learn if you don't have an experienced Helmatic coaching you. But once you know your way around the different files and directories that make up a Helma app it's very easy.
All in all, Helma is the closest thing to a useful, convenient Lisp web application server that I can imagine.