The National Geodetic Survey has a set of links online conversion utilities for performing some basic geodetic conversions, including:
- High-accuracy state plane coordinate system (SPCS) conversions
- UTM/USNG/geographic coordinate conversions
- Magnetic declination
- Surface gravity prediction
- Azimuth/distance to a second position (or second position from azimuth/distance data)
- Position from a dual-frequency GPS data file
Plus many more. Some of these are also available as PC programs, but these tend to be old-school DOS-based conversion utilities, not always the most user friendly.