Backup On Tap

I have to say that my external hard drive is one of the most useful things I have ever purchased. I use it to back up my data daily or weekly and I know that if my laptop ever fails I'll be able to switch to another PC and carry on working. It's so useful that I keep it plugged in most of the time . OK, it takes up a port, but that's nothing compared to the convenience of having backup on tap. What's your favourite backup solution?