Is there a table somewhere that shows how to convert between all the various rating systems?
US Chess seems to indicate that ratings would be converted from others to play in their tournaments.
I assume that you're interested in conversions involving USCF ratings. Here you can see the algorithm used.
As an example the conversion between FIDE and USCF is USCF = 180 + 0.94*FIDE if the FIDE rating is under 2000 and USCF = 20 + 1.02*FIDE if the FIDE rating is over 2000 elo points.
The official conversion rate for ECF to FIDE is FIDE=7.5*ECF+700, see
Again comments about this being approximate are appropriate.
I don't know of a more general table.
This tool converts between USCF and FIDE ratings: