incrediBILL - 4:11 pm on May 26, 2010 (gmt 0)
Sometimes you just need a specific recommendation, or sometimes you want to give a specific recommendation, that really would be helpful.
Primarily, links to free open source tools are generally acceptable and other helpful sites as long as they're authoritative on their topics and it's not obvious self-promotion.
Mind you, the forum charter may allow or disallow certain types of links and the moderator has sole discretion to permit them, unless the admins come along and supersede which is rare but known to happen.
End of story - no spam, no self-promotion, no sigs, all other links are discretionary.