[uptown]

Check out Koken. Pair it with Lightroom for editing if desired. Self hosted and quite powerful.

http://koken.me/

[icebraining]

Unfortunately not under an open source license, and the license can be revoked or changed "at any time, for any reason, without notice."

(EDIT: clarified that the issue is the license; the source is readable)

[uptown]

So basically it satisfies all of trbfred's specified requirements.