I have several active implementations now (in active use for the past 2 years), I guess I could easily water some of those down into a few examples.

With regard to similar systems... Good question. Comparable solutions:

- Ruby Passenger daemon. Almost feature parity with the Enterprise version, except for the "cool robot logo".

- Python Flask framework.

- Apache with mod_php or php-fpm.

- Roxen.

With respect to procmail, yes, indeed, and I picked up its maintenance again, and released a new version recently.