I have tried most if not all of them (free and paid) and two stand out - far above the rest.
The first is equalizer by Audioforge Labs - i believe it is $1.99 it is a full seven band parametric equalizer. What makes it fantastic is that you create the EQ curves by drawing them with your finger. it has a great decoding engine and plays FLAC as well. you can save and share curves you create with others. The interface is excellent and the sounds is superb.
The second and my favorite is Accudio by Goldenears.net. it is $4.99 and worth every penny. As you may or may not know goldenears.net is a Korean site (with an engish version) that does sophisticated measurements of headphones and earphones. Based on these measurements they create correction curves that can make a $35 IEM (the apple dual BA and JVC FXC80 to be exact) sound better than most $300 and $400 IEMs. I know this because I have experienced the magic of this app with these IEMs. They have over 300 IEM and headphones in their database and move being added each week. The app, as well as the support button on the Accudio web page lists the corrected quality of each IEM or Can (from 1 to 5) - so you can buy the cans that get a 5 like i did in a few cases - and boy was it worth it) It also allows you to tweak these curves, it also has a 10 band parametric equalizer (but you use numbers and buttons to create the curve - no drawing) and a clone mode where you can make your cans sound like high-end Byers, Sens, etc. The interface is really good, however the app developers are still on their first release of the app and there are a few nits they are addressing in the second version - right now it burns battery if you leave it "playing" after the music stops. a small price to pay for truly amazing sound quality.
I believe this kind of app is the future of portable, high quality sound where measurement based high quality digitial signal processing is coupled with high bandwidth transducers. The transducers may not have a perfect response curve out of the box, but the DSP system can correct it and you end up with a whole audio chain that is vastly superior to the sum of its parts.
So for an investment of $7.00 (which would not even get you a listenable set of IEM with one exception) you can turn all of your modest or even high-end IEM or headphones (the corrected CAL! is the best on-ears headphone sound I have ever heard when couple with this app and I have over a dozen high-end cans to judge by) you can go from good or great sound to in some cases simply sublime.
Feel free to ask any questions about these or other EQ apps - again I have tried most if not all...
Thanks for sharing mate, checking out the apps as we speak