We know from the FAQ on self-promotion that it is accepted only in a limited way in answers and frowned upon if found to be spammy. We know from this question that it can be deleted at any time from comments if the comment is not relevant or spammy.
But what about the chat? This leaves chat to be the only legitimate place to promote products or services that the user is affiliated to (albeit in an ad-like way).
What is your opinion?
The difference between the various rooms in chat goes way beyond the name of the room, each one has a different community inside. In some rooms, half of the participants swear like sailors, in other rooms letting a bad word slip is considered to be extremely poor form.
Some rooms love animated .gif files, others hate them. Some rooms tolerate a mix of languages, others prefer that everything be in English.
Self promotion in chat is not unlike anything else that is potentially taboo. Treat it like you would any other potential faux pas in a social setting - get to know those around you first, then gauge what's appropriate.
A friendly, on topic (as Martijn Pieters noted) link sent to someone that asked for a recommendation is generally going to be fine. Repeated links to the same thing may or may not be fine. Dropping into a room, planting a link and leaving is never going to be fine.
There's really no blanket answer here other than take the time to observe the community that you've joined, let them get to know you and then use your best judgement. When doing something that you're unsure of, be especially observant of criticism.
As long as it fits in the Chat FAQ topic guidelines I don't see any problems with linking to products and such:
This site is an extension of Meta Stack Overflow, so discussion should more or less revolve around the same topics you'd find at Meta Stack Overflow — but in an interactive, less strictly Q&A focused way. Do have fun, but please keep it professional and always be respectful of your fellow community members.
There is a fine line there, but if you keep it professional and respectful, promote away!
Do take into account that each and every chat room is it's own micro-community, with its own rules as to what is and is not acceptable. View it like any other social setting; walking into the room and shouting out a promotion of some sort is not suitable at a kids birthday party, to give you an extreme example. Chat rooms are the same.
