Shiro Kawai's SRFI cross-reference, which is part of his excellent Practical Scheme web site, is an excellent index of which Scheme implementations support which SRFIs.  It's much better than the one that used to be hosted on the SRFI web site, so I've linked to it on the SRFI home page.  Best of all, it's a wiki page, so everyone can help keep it up to date.  (For example, I just updated the entry for MIT Scheme.)