If you use Firefox there's also Rikaichan.
You have to be a bit careful when using automated tools like that as in some cases there is more than one possible reading. For example, 生物 pronounced せいぶつ means a living thing, and 生物 pronounced なまもの means raw/unprocessed food.
You might want to look into the sort of study sheets Japanese students use (漢字プリント or similar search terms should bring some up to print out, or there are various games online or for things like the Nintendo DS), which test you on giving the correct hiragana for kanji words.