NGS Coordinate Conversion and Transformation Tool (NCAT)
From the National Oceanic and Atmospheric Administration website you can find simple online converters for some of the more common conversions including:
NGA Geographic Translator (GEOTRANS) Tool
The National Geospatial-Intelligence Agency (NGA) Geographic Translator (GEOTRANS) tool download supporting multiple formats and platforms.
Klassen Code Library
Developed by SharedGeo's Jim Klassen, usng_tools is a freely available code library for developers that supports converting to and from USNG/MGRS and Latitude/Longitude (additionally UTM and UPS conversions are supported in the appropriate zones). Full details and code are available for download from GitHub.
Dolbow Google Mashup Code
Developed by SharedGeo's Mike Dolbow, USNG Google Mashup code is freely available. It relies upon the Klassen code library and the Google Maps API.
Magnetic Declination Diagram Service
Free utility which will generate a PNG or SVG file representing a properly formatted declination diagram for a specific map.