I frequently write about two ongoing themes: 1. the importance of "in the river marketing" (reaching targeted users at relevant points in the product / experience) 2. the difficult of driving mobile downloads from web, advertisement, other devices, etc

Here is a good example from Sparrow. They want to promote their popular Mac mail application to iPhone users. Within the initial product walk-through (now very popular within applications), Sparrow highlights their Mac app (Got a Mac?) and, to drive conversions, offers to send a download link. That's simple but effective (it's actionable), intelligent (captures some data / funnel measuring) and relevant (iPhone users are more likely to be Mac users than Android users).

Of course - if Sparrow were promoting their mobile product, SMS is more effective than mail. Groupon and Redbox do great work here.