Take a look at Trustive - http://www.trustive.co.uk/
They have wifi access in 130 countries. There are several different plans available - if you're only going to be using it on holiday they have pre-paid plans for 2 days, 1 week, or 1 month (from £12 to £29). They also have monthly plans for more frequent travellers. They also give you a program to put on your laptop/tablet/smartphone which finds the nearest wifi hotspots for you. They also do wifi + 3G accounts.
If you are a BT or Sky Broadband user, you may also have free access to wifi hotspots, but these are limited to the UK only.
But the trustive sim costs £55 plus £12 delivery before you even buy airtime 
for one off short trips abroad, I'd rather go and have a coffee or bar...not sure free wifi is enough to get me into a McDs 
The prices I quoted were for wi-fi access, which you don't need a SIM for. When I went to Tenerife a few years ago I had a Trustive account, and I used it in several wi-fi hot spots on the island, as well as in the departure lounge at Stansted Airport.
