How about sentence scrambles, where you write out a sentence on scraps of paper? The students then work in pairs to unscramble the sentences, race to the board, and write it correctly.
Or Chinese whispers as a warm-up. The last person has to tell you the sentence, and if wrong, check back along the chain to find the mistake.
Or any interview or bingo game, where students try to talk to everyone as quickly as possible, speak for as long as possible, make as few mistakes as possible.
These are only a few ideas off the top of my head. I like competition, but I also like putting together activities that require students to work together. I think for the weaker students, as well as the shy ones, this creates a positive, helpful atmosphere. Outside the classroom, they'll be less reluctant to speak, mistakes and all.
Take a look at MES English, too: http://www.mes-english.com/