What: According to founder Amanda Freeman, SLT is like the "Type A-version of Pilates" -- a combination of Pilates, yoga, and cardio on a Megaformer (think: souped up Pilates' Reformer). SLT is the latest impossible-to-get-into class, probably because it's also one of the hardest workouts on offer in the city. Class is intimate -- each one maxes out at 10 spots (creating even more demand) -- so you get plenty of personal attention. The instructor takes you through a 50-minute series of choreographed movements, and you do them all while balancing on the platform of the Megaformer. You're constantly using your core to maintain your balance, and exercises are designed to exhaust certain muscle groups. (Think: 15 minutes of non-stop abs.) That's what makes SLT special: it takes fitness moves you know well, and it makes them absurdly challenging.

Where: Studio in New York (If you're looking for a similar super-reformer class, check out Wundabar in Los Angeles)

Why: You'll increase your heart rate and sculpt lean muscles without getting sweat-drenched like you would in a spin class.

Who: Pilates-lovers in need of a challenge, barre devotees, women who are afraid of weights but want muscle definition, and perfectionists

What: Call it cuckoo, but repeating phrases like "I am strong" or "I am lean" while completing an aerobics sequence is actually effective. There's nothing like chanting at the top of your lungs with complete strangers to get you motivated -- and I'm not being snarky. When I took the class (at 11 a.m. on a Wednesday, mind you), it was packed with so many high-energy people. While I normally balk at any sort of cheesy group participation, the instructor Patricia Moreno was like a down-to-earth cheerleader who inspired us to move to our fullest potential, keeping the class engaged with a set routine of martial arts, aerobics, yoga, and dance. They also offer more specialized classes like "WarriorSati" (martial arts technique), "Soulyogasati (yoga-emphasis), DanceSati and CoreSati.

Where: Select New York Equinox Fitness locations, or IntenSati DVD series

Why: It's a peace-and-love version of cardio kickboxing or boot camp. You will sweat, but you'll leave class with a smile plastered on your face.

Who: Yogis who like to flow, the martial arts-curious, and the self-help/inspirational quote set

What: Drummers always have great arms -- so it makes sense that adding some rhythm to your workout will tone your guns along with the rest of your body. POUND combines non-stop drumming with cardio, strength training, and Pilates with a super soundtrack of rock and hip hop that perfectly match the interval and fat burning sequences. When I tried the class, I was surprised by the intensity of the movements, the serious core strength required for every step, and how difficult it really is to keep drumming on the floor. The best part: super-cool instructors who are upbeat rockstars ... and the fact that pounding the floor repeatedly can almost count as your daily dose of therapy.

Where: 21 states and 3 countries at select gyms, including Crunch, and dance studios. You can stream live POUND classes online for a fee at Poundfit.com.

Why: You'll sculpt your core, and nonstop drumming is extremely cathartic.

Who: The too-cool-for school set, music lovers, and barre class types who also want to sweat

What: If you thought Warrior III on the ground was tough, wait 'til you try it on a paddleboard. Adding water to your yoga practice forces you to work your core stability, and just being on the water will make you feel calmer and happier flowing through your asanas.

Where: Classes are offered near oceans and lakes throughout the US. Examples include YogAqua in Marina Del Rey, CA; SUPfari in Cape Cod, MA; Paddle Bliss in San Diego and Laguna Beach, CA; KamaDeva Yoga in East Hampton, NY; and Paddle the Mitten at several locations around Lake Michigan

Why: Who doesn't love a sun and water workout? (As long as you can avoid getting wet.)

Who: Athletic yogis, former dancers/surfers/swimmers, and adventurous types who aren't afraid of falling

What: Just in from Paris, Aqua Cycling takes your spin class into the pool -- promising a tranquil environment, and a more efficient workout with stationary bikes submerged in four feet of water. But this isn't your grandma's water aerobics class. Aqua Cycling is like SoulCycle transported to the spa. The boutique workout promises to reduce cellulite through the active massage created by water friction and muscle movement. And with the low-impact, water-based workout, you're less likely to get overly sore or strained. It's perfect for people trying to relieve any joint injuries, as well as pregnant women who want to stay active. But low-impact doesn't equal low-energy. The class is a killer workout because of the water resistance your legs have to push through while churning the pedals. Dress in your chicest one-piece, and the studio will provide special rubber shoes.

Where: One studio in Tribeca, expanding to LA soon

Why: Because Parisians know a thing or two about svelte figures (and cheese).

Who: Swimmers, spin addicts in search of a new medium, anyone with muscle or joint injuries