Thanks!
I've got it working..

---
require "open-uri"


print 'Skriv in adress: '
addr = gets.chomp


	open(addr) do |addr|
  	fil = File.open('file', 'a')
	fil << addr.read
	fil.close
end
---

Is that a good way to write it?
I also saw that the first "word" in the file 'file' was
#<StringIO:0x25350>.. What does that mean?