I've had AlternativeTo.net recommended. Looks interesting, and it's multi-platform.
"AlternativeTo is a new approach to finding good software. Tell us what application you want to replace and we give you great alternatives, based on user recommendations."
Update: Maybe it has some holes. For instance it had no recommendations for iPad Safari alternatives at all, despite there being half a dozen browsers for the 'pad.
So I googled for reviews, and found Perfect Web Browser, which seems very promising indeed (my post about it).