The “aveaword” campaign by Glue was a big hit for Mini in the UK. The live site is down so you can’t send the message on any more but you can create one and preview it here.
More recently Glue launched this site for Adidas.
I like that these are funny, short, personalized, novel, and the whole experience is send-to-a-friend. Lots of microsites that appear in the trade press online seem to me to be kind of long, involved and of dubious value to the user. These ones are a quick, easy and amusing enough distraction to be worth the effort on the part of the user, and for the client the brand message is conveyed clearly and simply.