Blizzard Downloader (or Background Downloader) was a combined BitTorrent/HTTP client used for Blizzard Entertainment's software products. It was first implemented with the MMORPG World of Warcraft. It was used to download the full game installer package, and periodic updates for the game. It was also used to download the trailers for The Burning Crusade, StarCraft II, and Diablo III. Early versions were based on the source code of the original BitTorrent client, though it is unclear what the source model later was. When a large update was issued, Blizzard Downloader was downloaded to the WoW directory and fetched the update file, using TCP ports 3724 and 6881. Blizzard offered an individual client for each download they offer, instead of just a torrent file. Blizzard Downloader also ran in the background under the name of Background Downloader. While the downloader was running, the program also uploaded parts of files over the internet to other users who needed the same data.
In patch 3.0.8, the look of the downloader was upgraded to match the launcher and made to look more specific to World of Warcraft. Previously, the window was much simpler with considerably less artistry. There were no obvious functional improvements, but the splash art could possibly change with each patch or for different product downloads.
The downloader was further changed with patch 3.2.0.
The Blizzard Downloader bundle was actually a plain BitTorrent client that has at least one .torrent file inside. The .torrent could be opened by other BitTorrent clients giving the user greater control over how the download is handled.