yum search "search-string"
will search any of your configured and enabled repo's, though its not always handy to use yum / rpm commands if your stuck on something like a .so.* dependency failure.
EDIT: I know the above isn't web, but there are yum GUI's around that emulate that function, I just can never remember a good one so prefer the command line.
For dependencies and as a general goto for rpm searching I use: http://rpm.pbone.net/ click the advanced search link to narrow the search to your distros.
I find this an awesome resource as it doesn't just index the redhat/fedora/suse default repos, though I'm not sure if it includes RPMFusion (which is another great 3rd Party source and is searchable).