Stationary Lunge
What it strengthens: 
Primarily quads and glutes with help from hamstrings, calves, and core

Cassandra's Note: 
Lunges have many functional benefits (balance, strength) as well as aesthetic ones (more muscle tone and size as well as a higher and tighter butt). Even though I know they can be tough, the benefits are proportional to the work!  

While standing, take a big step forward so your legs are about two feet apart front-to-back and about hip width apart. Your back heel will lift off the ground and most of your weight will be in your front leg. Bend your back knee down to the floor right beneath your hip. If your knee is right under your hip at the bottom, your feet are the correct distance apart. If not, make adjustments now. Both knees will bend to as much as 90 degrees, with your front knee behind your front toe and both knees and toes facing forward. Think up and down, not forward. Press up to start and repeat. Do both sides.

To make it easier: 
Hold onto something, like a chair, rail, or countertop to help with your balance and/or do not bend as far and/or do less repetitions.

To make it harder: 
Go slower and/or do more repetitions.