Leech is a nice download manager that integrates itself with Safari and Firefox (through FlashGet). There is a free trial you can try out and it costs $14 for a license.
jdownloader is really slow and sometimes freezes your computer, but it does the job. Just disable the unnecessary graph and it should run a bit faster. (Oh, and don't try to download 15 files at once on a 100mbit connection, your computer won't like it )