There are three main mobile providers in Portugal - TMN, Vodafone and Optimus.
They all offer PAYG (Pay as you go!)
At present they do not require registration. you can just go in and pick one up.
If you are buying a sim you must make sure you have an unblocked mobile as even vodafone portugal sims will not work in vodafone mobiles from other countries.
Vodafone tariffs here: (in Portuguese)
https://loja.vodafone.pt/tarifarios/vitaminas/Vita+Light+Total.htm
for the others have a look at
www.tmn.pt
www.optimus.pt
