I have used a few download managers in Joomla, such as DocMan and jDownload. But my lastest favour is Phoca Download. First of all, that is a GPL component and Joomla 1.5 native. Moreover, so far, that is very stable application. I have not found any bugs at all. In addition, that is only a component. It has plug-ins and modules. According to their documentation, they has some plug-ins can add the links to a file, category and section to an article. I have tried their module menu. That is very good. It can draw a menu to all sections in my homepage, that is good for the user browsing the by sections.