What are you looking for? There are for all browsers extensions that add a different UI to the download manager. The essential features are all in the modern browsers.
If you need more, JDownloader downloads just about anything from any server and processes entire lists of download links. It also works well together with various flash video download plugins in browsers.
µtorrent is usually a good choice for all torrent related downloads.
So called download accelerators are a thing of the past. Those used to be a thing when servers still limited what they push to one client over a single connection. They don't do anything at all in todays internet infrastructure.
Most of these are just external download managers. Just a browser extension works just as well.