My main online study tool up till recently has been readthekanji.com and I have written a little about my experience with RTK. However, I have recently started using another website too, by the name of KanjiDamage.com.
Kanji Damage is a bit unique. You learn kanji based on the kanji radicals and are given kanji compounds which are rated by their usefulness. This site, unlike Read the Kanji, is not really all that interactive. But for me, it fit's perfectly with RTK. RTK offers a flashcard style study method and one checks their ability to read a kanji or word by entering the answer using your keyboard. KD on the other hand is a completely static site, merely providing information which you can use how you will.
I have started his site to practice my writing (while at the same time, reviewing kanji and word reading). For this, the one thing the site is missing is the stroke order for writing. However, with a basic understanding of how to write kanji, you should be able to get close enough. If you really need to learn your kanji stroke order correctly from the start, this may not be the site for you. Alternatively, keep a kanji stroke order book at hand.
What I like about KD is the little stories he gives you for each kanji. This is where the site really stands out in its uniqueness. The stories are a little off, and could even be offensive. Personally, I find them kind of humorous. The reason I like this is that it gives you a way to remember how to write the kanji. For example:
- spring (as in hot spring) or 泉 is made up of the radicals (and kanji) 白 (white) and 水 (water). The very tame story offered here is that "The pure ,white water poured out of the hot spring." If you think about this as you practice writing 泉, it gives you a way to better remember the little details of the kanji when you later have to do a cold recall. You might be unsure as to whether the top half of the kanji is 白 or 日, but the story about the white water will easily prompt you in the right direction.
So, in summation, Kanji Damage is well worth a look-over or kanji writing study. The stories, once you start using them, really can make recalling how to write the kanji a bit easier.