KNewStuff, the KDE library
Since KDE 3.3, KNewStuff is part of kdelibs and thus available to all KDE applications. It offers a download dialog and an upload dialog, and currently integrates with over a dozen applications. With KNewStuffSecure, it's possible to enforce digital signatures.
Generic application: khotnewstuff
For testing repositories, the khotnewstuff binary is shipped with KDE which calls the download dialog for arbitrary GHNS repositories, and can apply filters given on the command line.