These lids were inspired by, and designed based on measurements from lids like these: 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: protoparadigm.com/blog/2012/08/what-to-do-with-a-3d-printer-make-canning-jar-accessories/