A lot of people are happy to write a letter or recommendation on request, but don't like to write a "blanket" recommendation, unless the person was a star performer. Be specific about the job responsibilities the person held, your working relationship with him/her and how long you've been acquainted.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I have worked with <name of employee> for <period of time>. During that period, I was <job title> and <first name of employee> was <job title>. <She/He> was an excellent employee, with <positive attributes>. <Optional additional accolades.>
We were 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 <her/him>. Please feel free to call me at <telephone number> if you require additional information.