Edit: Following a suggestion by Mithoron I've renamed it to heterocyclic-compounds which is also more in line with the tags that we currently have.
Edit 2: I've also done up a tag wiki. Feel free to edit/criticise/comment/praise/worship.
I'll keep this short and sweet, since I've gotta head off in 10 minutes. The tag heterocyclic-chemistry seems to have popped up recently. I think that this is a good tag to have, there are numerous books written on the topic and I had an entire course on it. There are already plenty of suitable questions.
As far as I know, "heterocyclic" includes saturated and unsaturated rings, therefore chemistry involving piperidine, pyrrolidine, tetrahydrofuran, etc. would come under this tag. So would chemistry about pyrrole and pyridine.
I've personally decided that questions about pyrrole, pyridine, etc. should also be tagged with aromatic-compounds, since these are aromatic heterocycles. I don't really anticipate any objections, but if there anybody disagrees, please do drop an answer. I'm writing this to make sure that everything is transparent and that there's a record of this discussion.
Note that the previous paragraph is superseded by the following discussion: Functional group tagging - revisited where it was decided that aromatic-chemistry should be restricted to carbocyclic compounds.