This website, created by a guy named Bret Mayer (obviously), is an essential tool for anyone serious about learning kanji, and especially helpful for anyone looking to take Japan's official kanji knowledge test, the Kanji Kentei.
Mayer's site has a number of useful resources that make researching kanji words nearly painless: vocab lists, a 4-character idiom index at various levels, proverbs ranging from simple to complex compound structures, a list of terms with non-standard kanji usage, and more. He also provides memory hints and helpful explanations to drive home the meaning of kanji. For example, 顎を出す (ago o dasu), includes the memory hint below it: "So tired you can only extend 出 your jaw 顎 forward." Another example is the kanji word 鄙見 (hiken) means "a dumb thought." Below this, he provides the following explanation: "Used to criticize another's opinion, or play down your own."
Due to the amount of difficult and complex content on the site, advanced learners, those seeking to take the Kanji Kentei exams, and even upper-beginner or intermediate learners who have a high kanji proficiency will likely get the most use out of it. Beginner and intermediate learners shouldn't be discouraged from visiting though, as this is a great resource for digging into really wacky kanji. I'm looking at you, 飇風 (hyoufuu, meaning whirlwind). It should also be noted that the kanji section of this website is only one half of the content. You can also find pages dedicated to Japanese tea and some stories in Japanese and English accessible via the homepage.