Bicep curls are one of the easiest ways there are to increase muscle strength in your arms. There re several ways you can target your biceps with bodyweight training but the easiest one there is requires you do nothing beyond lifting your own leg. As with every bodyweight exercise you end up exercising several different muscle groups.
What's involved: Biceps (primarily), shoulders, core, abs.
Powered by: Biceps primarily.
Tip: By raising your leg as high as you can each time you also aid in the flexibility of the hip.