aria2 is a utility for downloading files. The supported protocols are HTTP(S), FTP, BitTorrent, and Metalink. It has a powerful segmented downloading ability, downloading a file from multiple sources and multiple protocols and utilizing your download bandwidth to the maximum. It supports downloading a file from HTTP(S)/FTP and BitTorrent at the same time, while the data downloaded from HTTP(S)/FTP is uploaded to the BitTorrent swarm. Using Metalink's chunk checksums, aria2 automatically validates chunks of data while downloading a file like BitTorrent.
aMule is short for "all-platform eMule-like file-sharing program", or, in short, "all-platform Mule". It's a port of the well-known eMule P2P client for the eDonkey2000 and Kad file sharing networks, supporting most of eMule's features. Additionally, it provides a small separate Web server and command line interface to connect to a running aMule via the network.
LUFS is a hybrid userspace filesystem framework supporting many "exotic" filesystems (localfs, sshfs, ftpfs, httpfs, socketfs, freenetfs, and nutellafs) transparently for any application. It can be regarded as doing the same job as the VFS (virtual filesystem switch) in the kernel: it is a switch, distributing the filesystem calls to its supported filesystems. However, LUFS filesystems are implemented in userspace. This would be a drawback for local filesystems where the access speed is important, but proves to be a huge advantage for networked filesystems where the userland flexibility is most important.
LibTorrent is a BitTorrent library written in C++ for Unix. It is designed to avoid the redundant buffers and data copying that most (all?) other BitTorrent implementations suffer from. The library is single-threaded and the client handles the select loop. An interactive ncurses client is included as an example.
nget is a commandline NNTP file grabber. It automatically pieces together multipart postings for easy retrieval, even substituting parts from multiple servers and newsgroups. It handles disconnects gracefully, resuming after the last part successfully downloaded, and caching of header data for quick access. It automatically downloads only as many par/par2 files as needed.