Bookmarkable advertising is a pet topic of mine and I'm glad to point to the most recent of the very few examples of banners ads whose shelf life is extended beyond a single impression.
Spongecell, a company that makes online calendars with social features, is launching Spongecell Ads. These are online display ads with widget-like functionality that allow users to share the contents of the ad with their friends and also, importantly, set a reminder for yourself.
The example above is an ad for what looks like a music event, but you can see how this format can work with any date-based information: promotions, coupons with expiration date, sales and so on. To quote from an email from Spongecell, "We’re adding our Add to Life technology to a standard IAB advertisement to make it easy for a consumer to easily move relevant content from an advertisement into the tools they use every day - Calendar, Social Network, Mobile device or home page - think of it as a bookmark for an online campaign."