Megabolt is a cross-platform download manager. It aims for all the features of current download managers, such as segmented downloads, pause/resume support, support for a large number of protocols, etc.
rsget.pl is an application designed to retrieve files from download services (like RapidShare, MegaUpload, and many more). It has a long list of features already implemented, and an even longer TODO list.
ItSucks is a Web spider with the ability to
download (and resume) files. It is also highly
customizable with regular expressions and download
templates. All backend functionality is also available
in a separate library.