How do we feel about this?
I agree we should probably concentrate on a single tag.
GNU ELPA, MELPA, and Marmalade are all ELPA repositories, so
elpa sound like the right tag name from a technical point of view. But I guess some users may not know it, so maybe
repositories is a better choice.
package-repositories is probably more clear, but also seems too long-winded.
Of course there are viable questions one can ask about specific repositories. But none of the current questions fit that criteria, so I'd imagine the few specific questions that come up can just use repositories.
elpa is accurate, but as Stefan points out, it might not be recognized by some people.
package-repositories is OK, but it can suffer from both weaknesses:
"Repository" can mean any collection.
"Package" can mean any library - not necessarily something usable by
package.el- so any collection of libraries. (There can also be confusion with other meanings of "package", besides Lisp library - especially Common Lisp packages.)
I would propose elpa-repositories. What should be prominant are two things: (1) "package.el" or "ELPA" and (2) "repository". And just "package" here is not enough - "package.el" is needed, if we don't use "ELPA".
There is one advantage of elpa over elpa-repositories: The latter can seem to be only about the repositories, in particular, about the differences among repositories, whereas the tag is really about anything to do with the Emacs package system.