This week's episode of How I met Your Mother mentions another online tie-in related to the show: www.notafathersday.com. Perennial womanizer and keeper of the Bro-Code Barney Stinson proposes a holiday for single men: Not a Father's Day.
As soon as I saw the episode I went to the web site, and sure enough, you can buy all the outrageous merchandise showcased in the episode from the store on the site. Made me smile that the show followed up so well on slapcountdown.com and The Robin Sparkles video (this is to say nothing of Barney's Blog on CBS.com which is also fantastic). These are very smart plays which are written into the universe of the show without being heavy-handed- it doesn't have to be increadibly complex webisodes and adventuires to give fans a little better chance to engage with the show and its characters.