Rapidshare and Megaupload are two of the biggest file hosting sites on the Net. Tucan Manager is a free and open source download manager to aid you downloading from both.
It also handles many other free file hosting services including 4Shared, Mediafire and more. To use it, simply add the links for the files you want to download and Tucan Manager takes care of them by checking links for validity, controlling download speeds, pausing and resuming downloads, etc. It won't, however, eliminate the annoying waiting times that files hosted on Rapidshare and Megaupload involve although Tucan does handle premium accounts if you've got one.
Tucan looks great as it's written entirely in Python and it works across platforms. You can even expand Tucan with plugins which basically expand the number of websites Tucan can work with. The great thing about Tucan is that it's very lightweight and brings together downloads from file hosting sites. However, it would be nice if it handled other downloads as well as it's a bit annoying using two different download managers. If you regularly download from Rapidshare and Megaupload, Tucan is convenient and slick download solution.