Is there an online directory listing upcoming scientific conferences? I am looking at organizing a small conference next summer and I would like by all means to avoid concurrent dates with any major or highly-overlapping conference on the same or related topics. I thought that it would be really useful if there was an online directory where I can check a calendar of upcoming scientific conferences and workshops (maybe sorted by field) and plan mine on a week in which it does not coincide with an already planned event. That way I would not be "competing" for speakers and attendants against the organizer of a different event. I would prefer to avoid "manually" browsing the internet to find this information out, so a centralized online service would be perfect for my needs.

Does such an online resource exist? If the answer depends on the discipline, I am mostly interested in physics, chemistry and materials science conferences and workshops.
The first resource that springs to mind is the COMS directory, which lists conferences in the scientific disciplines (physics, chemistry, maths, life sciences etc). I don't know how comprehensive the listings are, but I think most major conferences should be listed there. You can filter the search by subject area (down to subfield level, such as astronomy or condensed matter) and country.

More specific resources exist which only list conferences in a certain country or subfield, for example the IOP (UK physics only), APS (mostly US physics only) or Inspire-HEP (international, but mostly high energy physics only).

Another resource used in my field (cosmology) is the UK Cosmo mailing list, maintained by the University of Sussex. Organisers can mail conference and summer/ winter school details to the list, and it's not limited to UK meetings either. A similar mailing list for your field may exist, although the benefit of such a list is its narrow scope-- after all, people don't want their inboxes spammed with emails about conferences in other fields, so I would use one with caution. Another drawback is that not everyone may be on such a list in the first place.