Which format(s) for citation files should I offer for my references? As I want to make my publications easily available for citation, at my website I am offering, together with my references, a citation file. For the moment I am offering a .bib BibTeX file with every citation.

The question is, if it is recommendable to use further citation formats. I have seen that Google Scholar offers 4 possible formats: BibTeX EndNote RefMan RefWorks. Is it recommendable to offer such a variety or it is enough to use BibTeX?

Thanks.
My take on how to promote and make accessible your works for citation is as follows:  

For your global library:  

Pick a format. You won't get it always right. Mathematicians and physics will prefer bibtex, biologist and health-related researchers probably will rather have something that works directly on their word (EndNote). In any case, I feel that getting all the citations of an author from his/hers webpage is not that common. People will just pick those articles that are really relevant for them.  

If you want to, at least, provide those two and you feel comfortable adding javascript to your personal page, you could use something like enw-to-bib to automate the translation upon user's request.

For each individual article:  

If you don't want to complicate yourself, just add the DOI. Most journals have citation tools to download in multiple different citation formats. If you want to get creative, this same citation tools will provide the links for quite a lot of formats (I guess some journals will offer more options than others).

This option should be that much easier and allow for a more flexible way for your visitor's to actually pick whatever they are really interested in.