Is it okay to post a link to other forums, or encourage people to visit them? For instance, if you know a good site for retired alligator wrestlers, or left-handed Swedish lesbians, can you say, “Hey, here’s a site that fills all your left-handed lesbian alligator needs”, or is it rude to link to other forums?
It depends - left-handed lesbian alligater clubs tend to be a little on the militant side so you probably could not link to one of them…but atomic bipolar badger hockey sites are fine. So - use your best judgement.
And I think rules are all posted in the forum rules part 
Moving thread from IMHO to ATMB.
Depends on the intent. Linking to other boards to encourage arguments with them is not a good idea.
Generally if asked the question and you know the answer and it takes you to another website, okay, sure; it’s great when Dopers provide solutions, that’s what we’re all about, dispelling ignorance.
But if that’s your main purpose for being here – advertising other forums – no, hell no.
We routinely throw out spammers and consider those who use this community purely as a means of getting more eyeballs for their own sites as not worthy of being here.
Creative Loafing also sells advertising on all their sites so if you want to advertise your favorite site for wrestling alligators and left-handed Swedish lesbians the ad office would be happy to have your business.
I meant posting a link as just an offhand suggestion. Like, in the thread I started about synethesia, can I tell people about the synesthesia forum I belong to? I’m not trying to advertise sites or anything, just suggesting a place they might be interested. Or in Cafe Society, if I gave a link to a book forum. That’s what I’m asking about.
As necessary and as appropriate, of course, in context. If you have questions about a particular subject or forum area ask the moderators of the individual fora; the rules are slightly different in each place.