Why does my internet speed vary so much and is slow tonight?

Last night I got a nice fast download speed of almost 500 kilobytes per second, which is very fast for me. However, tonight, I am only getting 20k. Everything is the same, it is the same time, all the other computers, the ps3, and the Wii are in the same state as last night (turned off), and yet I get a really slow connection. Why does it differ so much, and why on earth is it so slow??? (Don't give any Windows related answers, please. I am using Fedora 10 Linux.)