I bought an external hard drive to supplement my 60gig on the laptop. It has its own power supply. Although this in a sence stops you from being totally mobil but you can save a lot of stuff onto it. If you are in or around your house then you probably connected to a power supply on the laptop anyway. This way more space can be kept on the laptop HD.
If you have a DVD burner then you can use this to store stuff aswell. You can get external drives that use power through the USB port so still stay totally mobil.
Do a google for the different types. Hope this helps a little.