Physique 57
Cost: $57 a month or $5 to $10 per class; physique57.com

Best for: Fans of barre and Pilates mat classes.

What it is: Each of the 15 workouts—there's even one with Health's cover girl Erin Andrews!—mimics a traditional barre class, using micro movements to sculpt and lengthen muscles. While in-studio classes at this boutique chain can cost up to $36 a pop, you can rent content for 48 hours for a fraction of that price. (If you pay for the month, you have unlimited access for 31 days.)

Why we love it: Peppy instructors and fast-paced workouts hold your attention. "I was pushing myself the way I do in a traditional gym class but didn't feel the usual worry about how I looked," a reviewer of the Thigh and Seat Booster and Arm and Ab Booster workouts noted. Bonus: You don't need a lot of space or equipment—typically dumbbells, an exercise ball and a chair.

Keep in mind: Instructors run through moves quickly, so if you're a barre newbie, you may find yourself hitting rewind a lot.

Crunch live
Cost: $10 a month; crunchlive.com

Best for: Gym diehards who like to try new classes.

What it is: Joining this site—which features 33 of the chain's most popular workouts—is basically like having a Crunch membership, but without the crowds. Choose between standard 30-minute sessions or quickie 15-minute ones, all taught by some of Crunch's elite instructors (think Michelle Opperman and Marc Santa Maria). What's more, you can search by category, teacher, duration or class name, as well as place videos into a "favorites" section, making it a snap to find your top-rated toning sessions.

Why we love it: While there's no substitute for being in an actual class, our reviewers thought this service came close. "I really enjoyed the Ab Attack class," one tester said. "It incorporated yoga poses and stretching in the last quarter, which was a nice addition and different from other ab workouts I've done." Thrill seekers will like that the flat monthly fee encourages experimentation.

RELATED: 24 Fat-Burning Ab Exercises

Keep in mind: Your regular Crunch gym membership doesn't include these streaming classes. Free trials are available.

Udaya Yoga
Cost: $12 a month or $5 per class; udaya.com

Best for: Yoga enthusiasts who want to go deep into poses or just get a quick fix.

What it is: Subscribers to this strictly yoga site not only have access to an online video library but also get to try five new yoga classes a week. The sessions—in a variety of levels—range in focus from body-part-specific moves (such as quad openers) to skills (like perfecting Wheel pose) to traditional sequences. Search by class length, difficulty, challenges or style to find the right flow for you.

Why we love it: The step-by-step tutorials make it simple to follow along. "Even if you aren't looking at your computer, you know what's going on and can keep up," one tester raved. The breathtaking scenery of the outdoor classes adds an extra dose of calm. Om yeah!

RELATED: Which Type of Yoga Is Best for You?

Keep in mind: Some of the workouts are pretty tough. Our testers appreciated that the yogis suggest other sessions to help master the basic poses. Free trials are available.

AKT inMotion
Cost: $8 per class; aktinmotion.com

Best for: Dance lovers who want a 360-degree workout.

What it is: Wish you could take a class with a celebrity trainer? Here's your chance. AKT inMotion streaming workouts are the brainchild of Anna Kaiser, trainer to A-listers like Kelly Ripa and Shakira. Each 10-minute video serves up Kaiser's signature mix of cardio, strength training and stretching; you get targeted workouts (upper and lower body and core) as well as dance steps to boost your heart rate.

Why we love it: "At first I thought, 10 minutes? That's nothing. But I was really sweating a few minutes into it," said a tester of the Upper Body Workout, who admitted she was grateful for the on-screen countdown clock. Added our Dance Cardio Workout guinea pig: "Anna's energy was contagious—I did this workout first thing in the morning and felt completely revved up."

Keep in mind: Have a range of weights on hand because these intense moves will fatigue your limbs quickly. "I grabbed only 5- and 8-pound weights," said one reviewer, "but then there were a lot of pulsing movements and I had to go running for 3-pounders."

DailyBurn
Cost: $10 a month; dailyburn.com

Best for: Anyone looking for comprehensive training plans (it offers 14) as opposed to one-off classes.

What it is: After you answer a few questions (height, weight, activity level, what type of workouts you like), DailyBurn—a video-content partner of Health.com—will suggest plans that run the gamut from dance to kettlebells  . For each program, you find out the trainer and the length of an individual session, as well as what the overall series is and who the workout is best for.

Why we love it: The selection is incredibly diverse, and the workouts deliver results. "It's often hard to find online classes that are tough but not ridiculous," said a reviewer of Power, Intervals and Metcon1. "These are at just the right level, where I feel challenged but not so much that I can't do the workout." Another plus: the Inferno HR series. It's HIIT-style and allows you to track your heart rate on screen in real time, as long as you have a heart rate monitor and download the app to your phone or tablet.

Keep in mind: The hard-core workouts come chock-full of plyometric moves—like jumping lunges. If you have bad knees, you may want to steer clear. You can try it free for 30 days.