Coordinate Master is a powerful coordinate converter tool that performs both interactive and file conversions. It supports over 1700 coordinate systems from around the world, including lat/lon, UTM and US State Plane. You can also add your own coordinate systems, including local grids defined with affine parameters.
Coordinates can be displayed and entered as easting and northing values, or as lat/lon values in decimal degrees, deg/min or deg/min/sec formats. A built-in zone tool allows you to find the appropriate UTM zone for a given longitude, and buttons beside the coordinate values allow you to easily view the position on Google Earth (if it is installed), or view/edit the MGRS grid reference for this location.
Additionally, the "More..." button provides access to more built-in tools, including: Geomagnetic field, Compute datum shift, Point Scale Factor, Grid convergence, and Convert units.