Be as specific as possible about the capabilities of the employee. A copy of a positive reference is usually appreciated by the employee as well.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
In response to your request for a reference for <name of employee>, I give <her/him> an unqualified recommendation. <First name of employee> was <job title> at our company from <range of dates>. During that time, <she/he> was an excellent employee, with <positive attributes>. <Optional additional accolades.>
We were very sorry to see <First name of employee> leave, but talented and capable people like <her/him> often move on to greater challenges. You won't be disappointed if you decide to hire <First name of employee>. Please feel free to call if you require additional information.