How to convert coordinates

Hello,

I am looking for an accurate (survey level precision of 5 decimal places)
Coordinate converter program that can convert from Lat-Long (either in
decimal or deg-min-sec) to State Plane Coordinates (California), and then
vice-versa.  I know of Corpscon, but I've heard that it is not accurate
enough for my application.

Thanks in advance for your replies.

Phil in Orange County, CA