Internet download manager alternative for OS X?
Not free but my favorite is Speed Download. I went through a few free ones and ending up buying this on a MUPromo special and it was worth every cent.

It supports resume, password protected sites, and can even FTP if you need it too. It also allows you to share files with other users on the network.

I have to admit, though, since moving to the Mac, my failure rates on downloads without a manager has drastically dropped if you're on a decent internet connection.