A simple text link in the external links section of a page would generally be more appropriate than a box or template. We already have policies regulating the addition of external links to articles, and these should be applied to Wikicities and Wikitravel in the same way that they are to any other site. Just because something is free content, or hosted on a wiki, doesn't mean we should give these sites any special consideration when deciding what is actually useful as part of an article.
I feel Wikipedia has way too many of these boxes already, and we need to make sure these don't proliferate by allowing any free content site to add their link in this proposed box if that link is not useful to readers of that article.
For promoting Wikimedia's own projects, the template is more justifiable, but even there, I would prefer if this was regarded more as meta data than as article content. Generating sister project links automatically, or having them as part of the interface rather than the article would be far preferable. Anthere made a request in bugzilla related to this last year (http://bugzilla.wikimedia.org/show_bug.cgi?id=708).
Angela
-- Angela Beesley Wikicities.com