Adopt the style of the game show Jeopardy; it is a great way to review any topic for any level or age group. Depending on the group size, the class can play as individuals or in teams, making the game easily adaptable.
Draw five headings on the board and then further divide the columns from 100 up to 500 points. When finished you should have 25 answers in total and five answers under each heading. The teacher should have written out a question that corresponds to each heading and point category, with lower point questions being easier than the higher levels. With really small children you can bring in flashcards instead of reading out the question to the group.
The team then chooses a category and value, the teacher reads out the answer (or shows the flashcard) and the first team to put their hands up can provide the appropriate question. If they get it right they get the points. If not, it goes to another team for half the points. The team with the most points wins.
Bingo is another game that is loved by children and adults alike and which can be adopted to practice and review just about any topic under grammar or vocabulary. Practicing irregular verbs works quite well in this format. You write (or elicit, depending on the level) a number of verbs on the board. You write the infinitive on the board but call out the irregular verb tense. Once the students have filled their cards you can call them out at random and the students cross out the words they chose. Like regular Bingo, the first to have their words in a row, X or four corners wins.
There are a number of variations you can use to adapt to your students level and ability. With higher level students you can read a story out loud where they need to listen carefully to hear their verbs.
With vocabulary you can get the students to choose their words, then with lower level students you show them the flashcard and with higher level you can read out the definition or a sentence with the vocabulary in use.
One of the best online resources for English language games is Dave's ESL Cafe (see Resources). Under the section "Idea Cookbook," there is a section titled "games," with a list of game ideas that other ESL trainers have submitted. Other great resources for finding games online are the British Council and One Stop English. One Stop English has both free games as well as games that are only available to those who subscribe to the site.
"700 Classroom Ideas" by David Seymour and Maria Popova is a classic book full of games and ideas for the classroom. It is broken up into themes and almost all can be done with no more preparation than reading the section before walking into the classroom. Other popular resources among English teachers include the "Grammar Games" series by Mario Rinvolucri and "Games For Language Learning" by Andrew Wright.