dmenu might be what you're looking for. It just checks $PATH, and it's independent of any particular WM. I use it with dwm.
I've finally managed to have a usable desktop installation on my
Thinkpad X61. I have X running, Firefox (I need to install the Flash
plugin) and wpa_gui running fine with dhcpcd in the background.
I have used Fluxbox for long time on various Linux distribution, but I
haven't managed to have it auto regenerate its menu on Netbsd (I can
expand that if necessary). The file structure is different from the
one on Slackware and other Linux distros and I couldn't reuse my
existing configuration files.
I am looking for a recommendation for a lightweight manager that can
update itself after installing new packages and doesn't list non
existent applications. I have also tried Joewm but unfortunately I had
the same problem.