So two things.
1) Growth Hacks only work if there not over used, so it's in no ones best interest to share them until they are saturated (ex: Emailing listings on Craigslist a la AirBNB).
2) The best growth opportunities come from the product, and after product market fix.
So, with those 2 things said - here are some that I've used.
1) Find a way you can have your users share something in the app. Ex: Confirm investments on Angel List, Embed code on YouTube. At Timely we allowed you to invite team members to share tweets. Clarity we allow experts to use their VIP link to give free consulting to friends via Twitter & Email.
2) Prompt users to share their experience. We've done this in many ways but some of the most obvious are:
a) Prompting user post call to share
b) Providing a way for users to earn call credits by inviting their friends. (both members win = $ credit)
c) Prompt to share their profile on social media
As for the stuff that's having great success, well as I mentioned above, if I told you, then it would get saturated too fast .. hence the "hack" part of growth hacking.
A hack is a 0 day exploit. After 14 days it's usually no longer an exploit cause it's been patched. So yeah, it's not about marketing, it's about creative approach to making your product grow on it's own, then find communities to exploit.