SG is having issues right now (can't post or PM, but could maybe 20 minutes ago). Anyway, for osx downloaders, curl is pre-installed. It's good for numbered stuff. Be sure to use the -O option. It has a good manpage.
For some amount of HTML parsing, wget is downloadable. I wouldn't mess around with DarwinPorts unless you've already got that. Instead, just get
Both require knowing how to use a command line. Instead get
this port, which was compiled on 10.5.4 and works fine on 10.5.7.
Now, does somebody have a decent UPloader for the mac?