I use wget in both linux and windows.
it usually comes with the linux distro, but if you're using windows to download the iso, have a look at http://studwww.ugent.be/~bpuype/wget/
it will resume a broken download, whether the connection is broken by your modem or a reboot (the manner is kind of irrelavent).
if you're using windows, there are other gui options, such as GetRight (not free), Go!zilla (freeware and ads), and I recall a programme called Antz that wil download a single file in multiple chunks at the same time, each chunk starting in a different place (why you'd want to do this is beyond me), but it does handle cookies and related anti-leech tech employed by some sites.
I recommend wget over these
there's also curl, but I don't use it.