Hashtagify.me and Ritetag are some that come to mind that I've used. Might be worth checking out.
This and for instagram there's also websta.me.
RiteTag has a fully functional 30 days trial, it's a pretty neat tool for Twitter, if you install the browser addon, besides that it shows you the best hashtags, it will help you write kick ass tweets on the fly.
It's not always the best idea to target the most popular hashtags. RiteTag colors the hashtags by popularity and other factors, there will be red hashtags (those are the overused ones, which you don't want to promote) and there will be green and blue hashtags with less popularity but still enough traffic, what you should promote.
I usually go to hashtagify.me and use "table mode" to see the popularity and correlation of hashtags, this way i will get more hashtags similar to what i was looking for, then i fine tune them with RiteTag. Ditch the red ones and use only the green and blue ones. After tweeting a bunch, i do a final fine tuning using Twitter analytics to see which hashtags received the most traffic.
These services are mostly free to use with limited capabilities, if you need more insight, there are paid services. If you do a Google search, there are several.