Anki is a free app where you can download and create your own flashcards to learn Chinese vocabulary
Memrise: after a registration you have access to loads of materials for vocabulary learning. Choose Chinese and give it a try.
Prepare for HSK (level 1 to 6) with lots of exercise. Unfortunately they just made this website paid. I am keeping it in the list because it had very good resources but I will keep hunting for useful things online free of charge.
LINE: vocabulary by topic with pinyin, audio and translation
Chinese i Lab
Digital Dialects: learn common words and phrases by playing games
★ On LingoHut you can learn vocabulary by category, listen to the words and phrases and play games to help you memorize
Quizlet - lots of exercise on various topics
China.org.cn: you find dialogues, idioms, reading materials, etc. All with pinyin, translation and audio.