I have never come across anything myself, particularly as ClearType uses subpixel rendering; that said CoolType shows that it is possible for applications to render text that represents very close the actual printed result.
(That old chestnut of Arial, Times New Roman at smaller than 12pt etc. looking rather dismal on screen vs. print out)
I would expect the limitation is more to do with how XP (and Vista I am presuming) renders the display, essentially as a fixed size bitmap, where as operating systems like OpenStep use Display Postscript and Mac OS X uses PDF via the Quartz 2D Compositor as the display engine. Mac OS X by far has the best screen typography of all since fonts appear on screen (so long as it's a compliant Quartz 2D application) just as they would printed, so 12pt Arial looks like 12pt Arial on paper etc. And no--I'm not being a Mac zealot here, just pointing out the underlying differences
I could have coughed this all up as a load of slurred baloney, but I think that is the underlying reason why there isn't an alternative to cleartype.
Maybe if MS moves Windows to use the XPS based display engine (XML Paper Specification) we could see something interesting in this realm and would love to know if there are indeed better than Cleartype options out there, but it seems that Cleartype is as good as it gets.
Have you downloaded and used the Cleartype Tweaker powertoy btw?
Vicky