I don't know about particular websites, but I've had pretty good success making my own flashcards using the software
Anki and then just making an effort to run through them most days. They're not too difficult to set up yourself, and there are tutorials to help, but there are almost certainly existing flashcard decks for common kanji if you just want to use or modify someone else's.
I based mine off the textbook required for my university courses, Kanji Look & Learn, since those are the one's I need to know for class anyways, but if you need a place to start,
JLPT Sensei has lists of kanji required to know at each proficiency level. I set them up to have one card show me the kanji and I say out loud the pronunciation, and another card show me the hirgana and I write the kanji on paper; both cards also have a hint button that will give me what the word means (I have a separate deck for vocab memorization). Not the most convenient way of doing things, but I've found it sticks better when I incorporate the physical speaking and writing, not just reading or typing, and making flashcards is in itself a form of studying.
I also recommend learning the radical components of each character, too. It makes figuring out how to write them way faster and can sometimes give you hints to the meaning or pronunciation.
Anyways, that's the system that's worked for me; I've reliably learned about 300 kanji over two years of serious studying, even with slacking off over school breaks, but you might find something else works better for you.
