Whatever the subject matter of your web site there is probably a whole range of newsgroups that would be appropriate for a brief announcement.
Prior to using newsgroups as a means to market your site you should spend some time "Lurking" in the newsgroup. Lurking is the process of reading the messages and getting a feel for the group. You have to ensure that your message is appropriate for this particular newsgroup and your message can be tailored for the audience.
People who share interests in a particular topic gather in newsgroups and forums. (Forums are also called message boards.) Participating in these groups is a good way to become part of an online community and to find prospects for your products or services.
Some groups have a high noise to content ratio and are filled with junk ads. But others, especially those that are moderated, often provide helpful information and interesting discussions.
By getting involved in such a group and offering your advice and knowledge you can begin to build a relationship with other members of the group.
Good groups frown on blatant advertising. In order to be effective in these groups, you have to contribute something or have information to offer on your site that is of interest to the group. You can also ask questions and stimulate discussions.
You can focus your marketing in newsgroups because you can always include a signature in your posts.
Many forums do not allow this -- or it is not the norm. Forums are more likely to be moderated than newsgroups and offending posts will be deleted by the moderator.