This is a nice warmer, which can be used for any class regardless of age or English level, simply change the content to make it age or level appropriate. In a nutshell, you'll be giving a piece of paper to each student with a colour, shape, word, whatever, and have to walk around the class looking for the student whose paper matches their own.
Get a piece of paper and fold it in half. For the purposes of a clear description here, we'll say that you're teaching 'clothes' to a class of kids. On one side of the paper, draw an an item of clothing (you can find clip art online if you'd prefer) and on the other side of the paper, write the name of the item of clothing. Now rip the paper in half. Repeat the process again using a different item of clothing and it's name. Again, tear the paper in half. Make enough matching halves so that you have one for each student. Now mix up these halves and bring them to class.
At the start of your lesson, hand out one half to each child. They now walk around showing their half to each other and trying to find the student who has the matching half. Once found, they stick together. If you have an odd-number of students, simply give one half to yourself and join in, making it an even-number. At the end of the activity, ask each pair to show the rest of their class their matching pair and to say the word.
You can have matching colours for very young children, shapes, verbs, half-words, complex words and meanings, synonyms, antonyms, whatever. This really is a multi-level ESL warmer for any class.