If you are looking for the easiest way to search for artifacts in the Central repository, go to http://repository.sonatype.org. Sonatype maintains a public instance of Sonatype Nexus Repository that can be used to search for artifacts by GAV (groupId, artifactId, version) coordinates, Class Name, or Checksum. Here's a brief demonstration of the search capabilities on http://repository.sonatype.org.
If you want to manage your own instance of Nexus and index your own hosted repositories, you can download and install Nexus. If you manage your own instance of Nexus, you can proxy other Maven repositories (including Central) and publish your own organization's artifacts.
When you host a Nexus instance and proxy a remote repository, Nexus downloads a Nexus repository index. This index enables Nexus to quickly search the contents of a Maven repository. Downloading this small repository index is all Nexus needs to do to search the entire repository.