Anki, Quizlet and Skritter

 
I have used Anki, Quizlet and Skritter.

Quizlet is mostly free...a paid subscription will allow you to organise groups if you have classes to teach and add pictures to the cards. There are also games in Quizlet which I enjoy too. I have used Quizlet yet it does not have spaced repetition...however it is much more fun than Anki.

Anki is fine but not as user friendly as the other two. As it is mostly free and works across platforms. It has a very large following, but usually people love it or hate it. I fit into the later category  for me it is just too much work to set up.

Personally much prefer Skritter. I have been using Skritter for about half a year now.  Skritter does have spaced repetition, allows you to learn stroke order, tones, definitions and pinyin.  Lots of fun as it has a gamification feel to the learning. You also get lots of progress reports for your learning. I bought a Wacom tablet in July this year so I could write with a stylus rather than a mouse. This helped improved my writing of hanzi significantly. Skritter for me was the program that made hanzi learning very enjoyable indeed.
Both Skritter and Quizlet are also available as Apps for iPhone and iPad. Slightly different interfaces, but have the advantage of learn anywhere.