Habitica (App (Free) + Website) - Habit tracker that revolves around RPG game mechanics.
Forest (App (Paid) + Browser Extension) - locks down your whole phone/blacklisted websites for a certain period of time. I use this for pomodoro sessions.
A Soft Murmur - Ambient white noise.
sleepyti.me - tells you when is the best time to both sleep and wake up given a certain time.
Arts & Creation
Quickposes - good for practicing figure drawing. Cycles through a database of images.
Posemaniacs - another website to practice figure drawing. Renders the human body (tissues only) in 3D using Flash player.
BioDigital - 3D human model with all of the body parts, muscles, and organs.
PHOTOMOSH - website for glitchifying your photos.
etymonline - online etymology dictionary.
Squibler - deletes all your work if you stop writing for a few seconds.
Philosophy and Psychology
Changing Minds - A wealth of information on psychology, philosophy, and sociology.
Logic Portal - Information on all things logic.
Aesthetics Portal - The philosophy of aesthetics.
List of Cognitive Biases - Mostly for reference.