(This alphabetical list is constantly being updated as I find new things online! Check back every few weeks. I also use it as my own reference library whenever I feel like using a new site.)
Blonde Ponytail - This fitness instructor creates her own workouts and posts them on her blog, and includes workouts specifically for home (or playground!), upper body, core, Crossfit and treadmill training.
Bodyrock - This terrific website updates frequently with short yet effective workouts that can all be done at home with minimal (or no) equipment. I use these workouts all the time and will always review what I use, and provide a link to the specific workout.
Fit in Heels - This fitness and yoga instructor has posted both short and longer length HIIT (High Intensity Interval Training) workouts. It will takes you weeks to work through them all!
New York City Ballet Workout - This workout is over an hour long. If you are not looking for a pilates/barre fusion style class, but rather something that is straight-up traditional ballet, this is a good link.
Pilates on Fifth/Ultimate Pilates Workouts - You need to sign up, but signing up and use of all basic workouts are free. There are always 8 free workouts, rotating monthly, so you never get bored of seeing the same things repeatedly. I have also taken group classes at Pilates on Fifth and suggest that if you are in Manhattan (even if just on vacation) you pop over to try out a class or two.
Workoutz - This site offers short tutorials on specific exercises and is useful both for exercise ideas and to learn proper form. Want to see someone perform a glute kick with a dumbbell? A reverse crunch? The eagle pose in yoga? Look here.
Zuzka Light - She used to work for Bodyrock, and now has her own Youtube channel filled with powerful workouts. She demonstrates most moves before starting the routine, so you are not caught off guard trying to do something that hasn't been explained first. She generally only posts one new routine a week.