Super VICMON by David A. Hook is a modified version of SuperMON by Jim Butterfield.  This machine language monitor for the Commodore VIC-20 works great on an unexpanded VIC.

Tiny Mon by the one and only Jim Butterfield is a machine language monitor for the Commodore VIC-20.  Instructions are included in the archive.

Tinymon v1.0 by Jim Butterfield is a simple machine language monitor for the Commodore VIC-20.  Although it's pretty simple, it works with an unexpanded VIC.

Included in the archive are instructions both on the disk image, and in PDF format.  The PDF version was pulled from here, which appears to have been pulled from Compute! magazine, issue 20.