Signing Savvy is a sign language dictionary containing several thousand high resolution videos of American Sign Language (ASL) signs, fingerspelled words, and other common signs used within the United States and Canada.
ASL Sign Bank is great ASL vocabulary resource and you can follow them on Twitter or Instagram where they post a new sign everyday!
Handspeak is a resource for learning ASL and it has an ASL dictionary.
ASL Core is a sign dictionary word bank of specific area
Texas Math Sign Language Dictionary is a word bank of math-specific vocabulary.
Atomic Hands is a STEM (Science Technology Engineering Math) specific vocabulary dictionary and word bank.
SignASL is on of the largest collections of signs. They have an app for Apple and Android. You can also sign up to receive the "Sign of the Day" by email.