Depends how paranoid you are/how much stuff you’ve got.
If you just want to make a backup but you’ve got enough space on your pc to keep storing it then just get a little USB external hardrive – I’ve got a little Western Digital My Passport with a backup of iTunes and photos (powers off the USB drive so no need for any power supply…).
If you want to get fancy and allow the data to be access on your network (assuming you’re running wireless or similar) then maybe a NAS is the way to go. If you get one with RAID1 capability it’ll have two disks that’ll make two copies of everything just in case one disk fails. Obviously that’ll not stop you deleting files, or a virus destroying them etc..
They are probably the two extremes (certainly costs-wise!). Probably need to think what you want it for and make a call.
Or, of course, start burning DVDs……