There is a selection of free lottery sites online at the moment. Some of these are specific to the UK and others are available for anyone to play. Generally you just register and then all members are entered in a cash draw and winners are paid if they login to that site on the day of the draw and make a claim. Most of these are funded by advertising revenue and are completely legitimate.
If you have a spare few minutes then it is worth checking each day to see if you have won anything. There are a lot of people who think that they are scams because they have not won anything themselves. However, these things are down to luck and some people may win a lot while others never win and that does not make it a scam. It is wise to be cautious though as there are obviously many different scam websites out there. There are ways that you can check to see if sites are scams though, before you sign up to the sites. Look for reviews of them on forums and blogs as well as payment proofs. These will be in the form of screen shots of Paypal payments or winning notifications.
Of course, you may still be suspicious, particularly as some of these sites reward people who get referrals and so they will do what they can to get others to sign up. However, it can be worth looking at what you need to do to sign up. You will usually find that you will need to give very little information in order to do so and so it is quick and simple to do. Any blog posts or forum interactions between members of the sites will also help you to get a feel for the site and the members and possibly site owner so you can make up your mind about them.
Most of these sites do not have very big prizes but they will have better odds of winning than a national lottery due to low membership numbers. If you are the type of person that likes winning, even if it is only $5 then it could be worth a go if you have the time. If you are only interested in big jackpots then it is not worth trying. However, some of these small sites will grow and they will then be able to offer more money. Some have rollovers as well, which means that you could potentially get significantly more money if you win.