memrise.com (also an app for Android and iOS) - basically an advanced flashcard site. It attempts to guess when you are about to forget something and serves you up the word, phrase, etc again.
Duolingo.com - Never tried it because my language of choice isn't available but I heard it is very helpful.
italki - This website of course! Find a language partner to practice with or a dedicated tutor/teacher. There is no better way to improve in a language than to actually speak it with natives.