Several independent torrent clients, most notably µTorrent and Azureus, have created “peer exchange” protocols that allow for clientless torrent downloads. By means of the now-standard DHT trackerless peer-exchange format, these clients (and others) communicate with one-another and ask for info about new peers to download from.
Long story short, they let you download torrents with dead/no trackers, and without communicating with a central server. Everyone knows the importance of trackers of course, they provide torrent clients a list of known peers to download from – leechers and seeders alike, as well as track the status of a torrent at any given time. The problem is, they disappear mighty fast.