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.

What is your startup or industry? More context would make it easier to give you a better answer.

I've tested nearly 10-20 common ones in the past few months, but what I've realized about growth hacking in general (other than the fact that the phrase itself is rather misleading) is that it should be a lot less about techniques and more around a mindset. You can significantly hurt your business if you just try things others have tried and gain traction that just isn't real and sustainable. I think that's why no one has really answered your question here for a few days to begin with.

Good luck! More than happy to chat about the specific tests I've ran in the past if you're still interested.