The Museum of HP Calculators contains information about most HP calculators. There is also an active discussion forum on the site.

The HP41 Archive is THE Web source for all things HP-41. All of the module images loaded to the 41CL are available on this site.

This HP Museum thread lists the custom overlays that are available at very reasonable prices from Michael Fehlhammer.

Geir Isene's site is the source for HP solution books formatted for loading to the 41CL.

The CLONIX-41 site is a source for an alternate way to transfer data to and from the 41CL.

The HP-41 MCODE Wiki contains information about the OS software in the HP-41 and MCODE programming.

The HP-41 Programs site contains a ton of useful HP-41 programs.

The Calculator Store offers assembled 41CL calculators, as well as assembly service.