Well, youtube filters any URLs you put in your comments so I think they consider it spam. But don't let that stop you from doing so You can drive a lot of traffic to your site with comments. The only problem is that they will most likely delete them at some point but until then, its free traffic. And if you meant URLs in the video description, thats not spam at all.
Many people get away with posting urls by using link shortening services like Adfly or Bitly. There are others you may want to look into as well, some of which offer revenue for every unique who visits the link. While Youtube will filter out complete URLs as spam, there's nothing wrong with posting a shortened version that is not automatically hyperlinked. You still get many hits this way if what you have to offer pertains to the video being watched or something sought after. The best way to get traffic to a site via Youtube is by uploading a video, in my opinion.
I think the community there would indeed consider it spammy. I don't think it will do much good anyway since the members will just almost instantly mark your comment as spam and it would result in the comment being hidden. It's probably better to just spread links elsewhere, or at least be the uploader instead so your link won't be messed with.
If it is related to the video, it might be acceptable. But most of the time, Youtube filters out irrelevant or malicious-looking URLs. Be careful about posting URLs on the comments section of Youtube or any other media sites for that matter because it might result into you getting banned or penalized.