Are there any free public services where I can send one point in a coordsystem and it returns the point reprojected in a specified coordinate system?
I will not be doing many requests, for now it is just for a demo.
Looks like MassGIS has one: http://lyceum.massgis.state.ma.us/wiki/doku.php?id=point_reprojection Looks like it is only in the ESRI, non-open flavor though.
cs2cs can reproject a point or even a file of points
This already exists on spatialreference.org, but it is not widely publicized. Pass in some GeoJSON of your point and out will come a reprojected point. This is used to drive the map in the existing interface.
If something starts pounding the heck out of it, expect that we'll turn it off. You have been warned :)
GIS Lounge has online and offline converters listed here:
I found nearby.org.uk but you need to register and it only supports a few coordinate systems and postal code queries.
Try Online CS2CS - it uses proj4 and allows you to select all known coordinate system and change parameters or define your own coordinate system.