aria2 is a utility for downloading files. The supported protocols are HTTP(S), FTP, BitTorrent (DHT, PEX, MSE/PE), and Metalink.

It can download one or more files individually or from multiple sources/protocols at the same time and tries to utilize your maximum download bandwidth (by using multiple threads and downloading data from HTTP(S)/FTP, while also uploading to the BitTorrent swarm).

Using Metalink's chunk checksums, aria2 automatically validates chunks of data while downloading a file like BitTorrent.

There are other alternative applications. But aria2 has 2 distinctive features: (1) aria2 can download a file from several URLs(http/ftp/BitTorrent) and (2) If you give aria2 list of URLs, then aria2 downloads them concurrently. You don't have to wait for the current download queue to finish one file at a time anymore. aria2 uses your maximum bandwidth and downloads files quickly.

There are also some applications that have the ability of segmented downloading. Typically these applications are first split file rage by the number of threads and download them parallel and waits all thread finish. No re-split takes place. Normally there is no problem for this strategy, but if one thread is very slow(i.e. one of the server is very slow), then you have to wait for it to finish.
aria2 can cope with this peculiar situation. aria2 can re-split segment all the way down to 1MiB. So you don't have to worry about the above problem. But you might complain: if slow server is downloading last 1MiB, then you have to wait for that none the less. The nswer is "no". In such case, aria2 cancels slow server and use the faster server to finish download. In other words, aria2 is clever and very strong in many situations.

Unlike Aria, which has GTK+ interface, aria2 provides command-line interface only. But GUI-lessness brings lower resource requirement. The physical memory usage is typically 3MB(normal HTTP/FTP downloads) to 6MB(BitTorrent downloads). CPU usage in BitTorrent with download speed of 1500KB/sec is around 6%.

It is being actively developed by Tatsuhiro Tsujikawa.