As Sony works to expand their system, they've introduced a few more pancake lenses. You can see the entire lens lineup on Sony's E-mount Lenses page. Of these as of 2013, there are two "real" pancake primes plus a collapsing zoom lens which reasonably could be categorized the same way.
The 16mm was widely panned (um, slight pun intended; sorry) by reviewers. For example, SLR Gear says: "It's an interesting little lens, but its price point and its performance are perhaps limiting it to strictly consumer application." The 20mm is new enough that it's not widely reviews, but a common presumption is that a second pancake so similar in focal length could only be to quietly answer those complaints. The MTF chart in the Sony info page linked above certainly looks much better, and unless you have a very compelling need for your lens to be a little bit wider (about 12° horizontally), it's probably worth the extra $100.
There aren't currently any third-party E-mount lenses which could be considered pancakes. It would be pretty silly to use an adapter to mount a DSLR pancake (for example, the Pentax 40mm f/2.8), because the size of the adapter would dwarf the lens, but adapters to Micro Four Thirds appear to be available and are pretty small (only adding 2mm), meaning you could use pancake lenses for that system if you don't mind manual focus. (That'd probably also be a little silly, but if you've got multiple systems, access to the lenses, etc., it could be an option.)