No personal experience but have seen people use Gleam.io, Rafflecopter and KickOff Labs (they have a viral contest theme).

Also see these related discussions:
https://growthhackers.com/questions/ask-gh-growth-hacking-giveaways-gamification-giveaways
https://growthhackers.com/questions/what-is-the-best-app-platform-for-social-contests-out-there/

I've used Gleam.io and it was fantastic! The founder, Stuart McKeown, is very responsive too.

I've enjoyed using Offerpop a number of times. Check it out https://www.offerpop.com/

Hey there,

A quick intro, I run Growth in Viral Loops(https://viral-loops.com/giveaways), our goal is to help people like you, run hassle-free viral & referral marketing campaigns and build viral loops in their products.

We are template based and we offer 3 giveaway/sweepstake templates along with 2 referral marketing templates. We got inspired by successful company cases and we created templates based on these cases. For giveaways and sweepstakes we're offering:
- The Leaderboard Giveaway, inspired by Viral Loops
- The Sweepstake Giveaway, inspired by Mailchimp
- Spread the E-book, inspired by Lemonstand

The main difference here is that we focus on the viral factor a lot so all of our templates have sharing options, this means that your campaign participants have to share with friends in order to win more.

Learn more here https://viral-loops.com/giveaways