Tribler offers some very interesting technologies; the latest version enables users to search and download files from inside the client. Plenty of other clients offer search features, including the ever-popular µTorrent, but Tribler's results come from other peers rather than from a dedicated search engine. Users can search and download content without a server ever getting involved; everything is done among peers, without the need of a BitTorrent tracker or search indexer.
This is a little ironic because it means that the actual torrent file aspect of BitTorrent is no longer necessary. Normally, users have to find the torrent file corresponding to the content they want to download. The torrent file contains the BitTorrent tracker URL which, as its name implies, keeps track of everyone downloading and uploading the content in question. More recently, the need for a tracker has been replaced by technologies like peer exchange (PXE) and distributed hash tables (DHT). A replacement for BitTorrent search engines and indexers had yet to appear, until Tribler 5.3.
Since Tribler is still a rather new BitTorrent client, there are significantly fewer torrents to search from compared to popular BitTorrent search engines, but as it gains more traction, that number should grow exponentially. Furthermore, if competitors like µTorrent jump on board, we may have yet another P2P revolution on our hands.