Is there any world-wide ranking of conferences/journals?
In Computer Science, CiteSeerX used to provide a Venue Impact Factor, that included journals and conferences, but it seems they've stopped doing it. As indicated by Gopi, Microsoft academic Research is some kind of global ranking, and JeffE also mentions google.scholar.

That being said, a global ranking, across sub-fields, is not necessarily meaningful. For instance, if one does not work on programming languages, then it's unlikely to submit a paper at POPL (the first venue according to CiteseerX). Hence, I'd say that field-specific rankings matter more, for instance in security: http://faculty.cs.tamu.edu/guofei/sec_conf_stat.htm or https://personal.cis.strath.ac.uk/changyu.dong/ranking.html.