These lids were inspired by, and designed based on measurements from lids like these: http://www.amazon.com/36010-Mason-Canning-Plastic-Storage/dp/B001ASXI8 . There is a regular mouth and a wide mouth version, these should fit any standard canning jars.
If printed well, and well tightened, these lids are almost water tight (actually, they form a tighter seal than the commercially sold counterparts linked above), but not air tight. These are suitable for use in storing dry goods, non-food small parts, etc., for storing opened canned goods in the refrigerator, and probably for use in the freezer.
The main shape of the lid is deliberately low-poly so it is easy to grip and apply a good bit of torque with bare hands.
These were printed with ProtoParadigm's green PLA on a Makergear Mosaic.
Additional accessories will be released that use this lid as the base.
UPDATE: This thing is a part of a collection of things featured in this post: http://www.protoparadigm.com/blog/2012/08/what-to-do-with-a-3d-printer-make-canning-jar-accessories/
Load up the STL for your canning jar (wide mouth or regular) and print. You may have to do some cleanup of the threads with tweezers, but if you clean up the really out of place stray threads (and the ones laying across where the lid contacts the lip of the jar), the small imperfections generally won't keep the lid from threading properly.
If you want to work with the openscad, keep in mind that render times are pretty long if you have the quality turned up on the threads. The comment on that line should give you an idea of reasonable values; the files released here are at a quality of 45.
This thing uses Thread Library by syvwlch (http://www.thingiverse.com/thing:8793) to generate the threads, so you'll need to grab that to render these.