RPM search engines
RPM search engines are search engines that allow you to search for RPM packages for software - and in most cases, allow you to search for the RPM that "owns" a particular file, and for the RPM(s) that "Provide" a given named feature (such as "webserver").
WARNING: Packages that are not built for your particular distribution, may or may not work. It depends on a number of factors, such as architecture, required libraries, naming conflicts... It is safest to try and find an RPM that is built for the exact same version of the distribution as you are running (e.g. SuSE 8.2). Also note that although OpenPKG packages are designed from the ground up to be cross-distro, they require an incompatible fork of RPM, a fork which uses a separate package database.
- Rpmfind - Runs the powerful rpm2html software, which supports searching for individual files within packages, and browsing by repository or distro. Mirrors: fr2 (Speakeasy - currently down) - Remember, mirrors are not always as up to date as the master site. However, if a mirror has the RPMs you need, it can often download them quickly while the master site is swamped with requests.
- RPMseek - Supports searching for individual files and Provides. A slightly more intuitive interface than RPMfind if you want to do non-trivial searches.
- RPM.pbone.net - Supports searching for individual files and Provides.
- Tuxfinder - This one can also search for tarballs
Distribution-Specific RPM Search Engines
These search engines focus exclusively on one Linux distribution. They include both "official" distro packages and "third-party" extra packages.
- FedoraTracker - RPMs built for the Fedora Core distribution only
Individual Repositories
In most cases it will be faster to use one of the search links above, which will search through multiple RPM repositories for you - unless you specifically want to get a package from a particular repository for compatibility reasons. Note also that most of these don't have search engines as such - they only support browsing for a named package.