There are currently six Transformers newsgroups. Three are for discussion, two for selling and trading toys, and one is for posting fanfic. The newsgroups and their topicalities are listed below. Take a look, and feel free to participate in any that you wish. A FAQ or guide for each newsgroup is available on the Transformers FAQ Homepage.
Six may seem excessive, but the creation of each new group did have a justification. Here's a little history lesson:
It all started in 1992 with an email distribution list called, simply, the Transformers Mailing List, formed by a group of fans who had found each other in rec.arts.animation. By the middle of 1993 the list had grown large and unwieldy, so to lighten the burden a newsgroup was created to replace the list: alt.toys.transformers. For several years ATT served as "the" place to be online for Transformers fans.
As the Internet's population grew in the mid 90s, the newsgroup started to become bloated. In an effort to make ATT easier to navigate by moving sales posts to a different newsgroup, alt.toys.transformers.marketplace was created in December of 1996.
By that time, the internet boom had truly begun, and there was a huge influx of people to the newsgroups. Eventually there were enough posts that the fanfic-writing sub-community was feeling stifled; with so much going on, it was hard to find fanfics among the vast field of headers, and even harder to maintain a discussion about it. In order to provide a more nurturing forum for TF fanfic alt.toys.transformers.fanfic was newgrouped in January 1998. The low traffic of ATTF is better suited to the creative process.
Later in 1998 talk began of splitting ATT yet again in hopes of solving two problems: A) many of the new netizens who were posting to ATT seemed wholly unconcerned with behaving themselves, and 2) Beast Wars had become so popular that there was little bandwidth left for discussion of G1 TFs, which understandably distressed fans who didn't care about BW.
In the spring of 1998 the command message was sent to newgroup alt.toys.transformers.classic.moderated, providing a forum for classic TF discussions that would be safe from spam and flamewars. Shortly afterwards, the call-for-votes was completed on a proposal to create a new set of Transformers newsgroups in the rec.* hierarchy, which has higher visibility and distribution than alt.* does. As a result, two more groups were birthed: rec.toys.transformers.moderated and rec.toys.transformers.marketplace, which are essentially "new" versions of ATT and ATTM. Unfortunately, the rec groups have not been very successful. While they are there and functional, they don't receive very many posts.
Copies of the creation messages for each of these groups are available in the "Tidbits" section of Steve-o's TF website. The creation messages are of mild historical interest, and also contain the newsgroup charters. Also possibly of interest is a story about the early history of online Transformer fandom written by Phil "Skyjammer" Zeman, one of the guys who's been a part of it from the very beginning.
If you are unable to access a newsgroup that you want using your ISP's newsfeed, the best thing you can do is write to your news or system administrator and request that the groups you want be added locally. If the group is available to them from their "upstream" site, they will most likely do this for you after only one request. You may need to nag occasionally, though. For many netizens, the news admin's username will be "news", and their domain the same as your own. So, if you are "joeschmoe@some.isp.com", your news admin is probably "news@some.isp.com". That's not universal, though, so if it doesn't work, do a little digging to find the right person to write to.
As an alternative to getting the group from a local server, you can set up an account with a remote news server, or use a web-based news service. The most popular web-based news is Google Groups, mainly because it provides free reading and posting. (It also has a fully searchable archive of Usenet from almost the beginning of time.) There are many other options, though. Below you'll find links to lists of news providers in the Open Directory.
This is a much harder question to answer than "What newsgroups are available?" Before the days of web boards, if you wanted to post messages about something, you went to Usenet. Usenet is nothing except forums, and it's easy for anyone to see a list of every last forum that exists there. Cataloguing web boards is all but impossible in comparison, because any website, even a small one, can host its own discussion area. There are dozens of active (ie. non-tiny) TF web boards, and an uncountable number of small ones. Pretty much every TF website that has its own domain name has a board system, for example. The best I can do is point you to a few of the biggest boards; the ones that I see people talking about. Please don't take this as an endorsement or condemnation.
As mentioned in the "Transfans" section,