I have used this for the past year with great success. It is called Brick-on/Brick-off.
Each table has five bricks of their own colour on their desk, e.g. red table has red bricks. I also have 5 of each colour on my desk.
When the children do something which requires a reward, they collect a brick from my desk (Brick-on), or if they do something which has a consequence, then they get a brick taken off.
At the end of the week, the table with the most bricks is the top table for the next week. They get privileges such as lining up first, getting the first choice for choosing time etc.
This is a really good reward system as it helps the pupils to work as a team and see themselves as part of a group. They can get bricks for group and individual efforts. This also allows them to see that their actions have consequences for others.