Most professionals are well versed on the need to acquire business references and LinkedIn recommendations, and hopefully are just as accommodating in offering them to business associates. Although, an often overlooked opportunity to acknowledge and support someone in their career is the classic "thank you" letter. These are especially meaningful to people outside of your network, and can carry a lot of weight at review time for the recipient, as they are unsolicited.
Managers of employees who are spotlighted for their efforts recognize the sender has gone to a lot of effort to get an email or letter in front of them, and place value on the feedback. Additionally, individuals can keep copies for their "kudos" file. Service workers are great candidates for thank you letters. We are often quick to complain about poor service, and maybe are pretty good about a verbal thank you, but taking the time to send a formal thank you is rare.
When sending a thank you letter, as with references, make specific points as to why you are sending the letter and what the employee did to deserve the acknowledgment. These should not be used for basic customer service, but for someone who has gone above and beyond, and you should describe the event in detail. My thank you notes are typically at least three paragraphs long, and are formally structured even when emailed. Try to send a copy to the individual also, as they may never know otherwise.
Here is a brief sampling of thank you letters/emails I have sent recently to provide some ideas: