Hacks have no fixed place yet on the mediawiki.org website.
They are lightly discouraged as they are generally tied to a small set of software versions, are not widely tested or portable and may cause unanticipated problems. Hacks that may be genuinely useful to a number of users should go on either
- the talk page of the related issue (e.g. if it is an installation hack you might want to put it at Talk:Installation),
- on the talk page of the related configuration setting
- on a new page discussing the issue and offering a workaround - however it is only worth doing this if the hack is presented with full context in a way that would be useful to someone experiencing the same problem.
If the hack is something that should be incorporated into the main MediaWiki codebase (e.g. a bug fix) you should create a new bug on Bugzilla containing your patch, rather than posting it here.