Preferably, the webiste/app should be free. Thanks.
You can take the Oxford Online Placement Test (OOPT) on italki to know your level, but it costs 100 ITC = 10 USD
http://www.italki.com/p/oet.htm?src=italki-footer
Good luck :)
Duolingo is the one I know of. But search "learn English" on the Apple or Play store, hit "Free" for the price range, and you'll probably find a dozen or more excellent apps.