FrostWire is a pretty cool program. You can use FrostWire to download music off the internet and do other filesharing such as videos and documents. I like to download ebooks off of FrostWire myself, but thats just me. Most people use FrostWire to download music and MP3s for their iPhone.
One time I was on FrostWire and I almost forgot to set my speed to low. I woke up the next day and found that I have downloaded over 10 gigabytes and my harddrive was almost all used up! If you have ever used standard LimeWire this probably wouldn't even be possible because they limit your downloads to only a certain amount. But with FrostWire you can download as fast as your internet will allow you to with zero limits. The best part about FrostWire? It's completely free!