I've been using perfectaudience.com and found great success. It's very easy to use and the results have consistently been above average. It's Facebook specific but where are most people spending their time? Facebook.

I recommend to all my clients that they use a combination of Google AdWords remarking, and Facebook Exchange retargeting (FBX). Google AdWords remarketing lists can be built seamlessly using your site's Google Analytics tag (as long as you've linked AdWords and Analytics accounts, and updated the analytics code ... https://support.google.com/analytics/answer/2611268?hl=en)

For FBX, PerfectAudience or AdRoll are both great options. I tend to use AdRoll more, as it offers some great other retargeting options too (via their extensive non-FB network of sites).

Shout if you need more help.
Mick

Top 4 Retargeting companies:
AdRoll
Retargeter
Fetchback
Chango

You will have to check individually with each network in their help files. They will give it to you as soon as you sign up.