[gsiener]

Mixpanel actually offers this service now: http://techcrunch.com/2012/10/22/mixpanel-engage/

I've used similar, standalone services and also built my own, but I'm excited to try Mixpanels' offering because it's already integrated into the tracking.

[jdangu]

My solution is probably 3-5 times cheaper though. It's actually free for me (KM via appsumo and Mandrill free tier)

[lingben]

mixpanel is free with a small badge link back