I generally will start with this codeplex version that is free. The author has had many releases with minimal issues. crm2011autonumber.codeplex.com
Please be sure to consider the method of the numbering wherein if it simply does a max count + 1 increment, then if a record is deleted you will receive duplicates. Also consider the transaction volume of new record creation (even including data migration efforts) if it allows for parallel action you can also get skipped values and duplicates. In these areas we usually inject additional code according to the business logic.
