Is there an online directory listing upcoming scientific conferences?
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.