Jeff Mangum was also a member and contributor of many side projects loosely affiliated with either Neutral Milk Hotel or The Elephant Six Collective, including Major Organ and the Adding Machine, Circulatory System, and The Olivia Tremor Control. Not long after the release of and supporting tour for In the Aeroplane Over the Sea, which was somewhat of a concept album partly inspired by the story of Anne Frank, Jeff Mangum disbanded Neutral Milk Hotel and since then has avoided public eye and has performed live only a handful of times. An article discussing Mangum's Barrett-esque dissappearing act can be found here. Recently there was circulation of a message credited to Mangum reporting that he would return to the spotlight in the near future, but ultimately the message turned out to be contrived.
Apparently if you are in hopes of hearing any new work by Jeff Mangum or seeing a live performance the mantra is "don't hold your breath." However, we can still look back and listen to the work that he has left with us, from which I have selected the following tracks. In each of the songs below Jeff Mangum is either the main artist, a contributor, or sometimes producer.
Additional Neutral Milk hotel live footage
Jeff Mangum on Wiki