These games work well in class with an interactive whiteboard, although students can also use them for self study on mobile devices or on computers (especially if you have a large class). In small classes, they work best with teams of about three students at the board taking it in turns, and a time limit. The rest of the class can watch (and shout out clues or distractors if you like a noisy classroom) before they take their turn. Most of the games are ready-made, but with some (e.g. Puzzgrid) you or your students can design your own quizzes.
This website tests vocabulary using pictures and there are a few different game settings so you can focus on different things, such as spelling or listening. It's great for learners at around A2 level - the vocabulary is a bit simple for higher levels. It's also fun for multilingual classes as they can test each other on their own languages - it's often surprising to see how many words are similar to English.
This is a great website which lets you test your vocabulary and supports the World Food Programme. It works up to even the highest proficiency levels: the questions gradually get harder and harder (you can choose the starting difficulty) - see what level your class can reach! If you click on the hamburger menu in the top left corner ( ≡ ) and select categories, you can choose different types of question - there is also a grammar quiz option among other things.
This British Council game is a good way to encourage attention to spelling and if your students enjoy scrabble, they should enjoy this too. Basically they have to make as many words as possible using the letters in the box (the letters don't need to be touching). They score more points for longer words.
In this game, you click on adjacent letters (horizonally, vertically or diagonally) to make words which then disappear. The objective is to clear as many lines as possible, so there is some strategy here too. This game is a bit more advert heavy than the other sites here, but it's really fun for small classes or for students to play at home. The parent website (Arkadium) has lots of other free games to explore.
Give the class a number of categories (e.g. animals, drinks, jobs ...) and a letter (e.g. F). They have a time limit to think of a word beginning with this letter for each category. To be honest, this is one game that EFL teachers have been doing for years without needing technology, but I do like this online version - if nothing else because you don't need to spend time trying to think of categories and some of the categories are quite interesting, complex and encourage communication - they can work for B1 levels and above.
There are lots of versions of this online. Again, this game doesn't really need technology, but the online versions make it look much more attractive than my handwriting ever does. I like this version because it's free and you (or your students) can put in your own word lists and save the links.
Similar to the BBC quiz show Only Connect, you have to make four groups of four connected words, from 16 words. Usually there is some overlap between the categories. For example, with a high level class who have studied prefixes, you could put in the group danger, large, close, rage (they all take the prefix 'en'). For lower levels, you could have groups such as fly, ant, bee, mosquito (insects). You can make your own (or get students to make them) on the puzzgrid website. They are easy to make, but remember to save the number of your puzzle - you'll struggle to find it again otherwise!