KHAN ACADEMY is unbelievable! You can learn anything under the sun on there. Plus they have all kinds of educational games for all ages.
LUMOSITY is super cool! It's like having a personal trainer for your brain. Visiting the site is like going to the brain "gym". Haha.
Dr. Bill Vicar's American Sign Language (ASL) Fingerspelling Practice Site is awesome for anyone who is involved with American Sign Language in any way. It's just fingerspelling comprehension (receptive) practice. You can choose different speeds and challenge yourself. I am a sign language interpreter, so I love this site! It has strengthened my "receptive" and voicing skills a ton!