If you want a smartphone rather than that archaic piece of technology that Nick posted, I've got a Nokia Lumia 520 that was £115 brand new & Sim-Free. You should be able to easily get it for less than £100 secondhand.
It's actually pretty damn good for the money too. But it does depend on what you're after. If you're big on apps and games then it's going to be no good to you. If you're after a phone that you can google shit on, get your emails, check facebook, watch videos, listen to music etc then this is spot on. It has a micro SD card slot, so for about a tenner you can get a 32gb micro SD and life is wonderful thereafter.
It's surprisingly speedy and just works. I am a fan.