SIDENOTE: is it still true that Apple doesn't document the menulet interface? (i.e., the Menu Extras on the righthand side of the menubar). I thought that all non-Apple things in that region were deemed "hacks" by definition.
There for awhile non apple menulets needed to be black and white only, only apple's could be color. And I think even now all non apple menulets have to be left of apple's. (and you can't move them as you can with apple's)
I don't recall the black & white thing at all, but MenuCracker (as per Hal) still enables the MenuMeters menulets to not only be moved, but to be moved to the right of Apple's, and
MagiCal still inserts its menulet just to the left of Spotlight (It can be moved, but only from left of Spotlight to extreme left.), but it needs to restart SystemUIServer to turn the trick.