I have DSL and I usually leave Limewire to download in the evenings when I go to sleep. I set my bandwidth limit to unlimited, but my download speed, no matter how many, always hovers between 80 and 100 KB. I've tweaked firewalls, opened ports, done everything possible, but I can't fix it. Any help?

Download speed depends on your connection plus the connections of the hosts you're downloading from. You can have your bandwidth set at 'unlimited' but you'll still only download as fast as your connection allows! If you're sure that you should be downloading faster, could it be that your ISP shapes your bandwidth for p2p apps?

You can do a speed test here
http://www.dslreports.com/speedtest?more=1

There are some good tips here to get the most out of your LW
Configuring LW for best performance

download speed is 720. like i said, it doesn't matter if i'm downloading 1 file or 100 files at a time, download speed always stays between 80 and 100

Is that 720 kbps? Divide by 8 to give you KB = 90KB which exactly what you're getting.