Jay-Z's Empire State of Mind relies on samples from both Isaac Hayes' "Breakthrough" and The Moments' "Love on a Two-Way Street". You can hear the originals, and dig into more sample science, at the WhoSampled web site.
Started by a part-time DJ, the site does a great job of showing you exactly where the reconstructed moments in popular songs come from. YouTube videos of each song are matched up next to each other, and the time at which the relevant sample starts is written just above each. Users can submit their own sample findings to the site, which are queued up and eventually posted and approved.
I've always found great hip-hop sample coverage at The-Breaks.com, and it probably still has a deeper database at this point. But the inclusion of YouTube video samples makes WhoSampled a nice resource for finding, and enjoying, older music that's being made new again. Free to use and register for.