The guest request system that I have in place has proved very popular with many clients.
When a client confirms a booking with me I send them a username/password that gives them access to the client area of my website. One section in here is the Guest request system, which gives them a link and a password that they can send to their guests to enable them to make some music requests.
The guests can browse the music library, select requests, and include special notes about the requests they make. The client is notified whenever a guest logs on and makes a request. After navigating to the link, the guests will need to log on using the date of the event and the supplied password.
The client is notified each time they make a request but they can turn these notifications off if they wish.
The client can also remove any tracks selected by their guests they would rather not hear!
Further to this on the night I always announce on the microphone that guests are welcome to ask me for any requests they would like to hear, unless the client requests that this not be announced!