Coordinate Master is a powerful coordinate conversion tool that supports 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.
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. The lat/lon format can be changed on the preferences page.
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).