This is a very useful site, but I do have one small suggestion that might help streamline it a bit:
The units are on the left side of the page and the conversion boxes are all on the right side of the page. In most sections, there are quite a few rows of these and I thought it might make the site easier to read if you could hi-lite or darken every other row to help delineate which conversion goes with which unit.
An alternative, of course, would be to simply print the units to the right of the conversion boxes for fast and easy referencing.