I use Big Fish games. They aren't free, but some of them last for months. I like the finding games, as MissChateleine describes, but there are other kinds on there too. I like the Vacation Paradise series, where you get various different scenes from the country in the title (I'm currently doing France) and find things in a variety of modes. There are basic ones where you are told what to look for (eg a giraffe or a football) and others where you are given clues, or anagrams etc and have to find them using those. Many games have a storyline, although I prefer the straight 'hidden object' ones. You can get games for PC, Mac or tablets.