Mathigon
Mathigon: Geometry: Circle and Pi: Degrees and Radians
Rather than dividing a circle into some number of segments (like 360 degrees), mathematicians often prefer to measure angles using the circumference of a unit circle - a circle with radius 1. This way of measuring angles is called...
CK-12 Foundation
Ck 12: Algebra Ii: 4.4 Radians
This section introduces radians an alternative to degrees as a way to measure the rotation around a circle. It also begins to draw connections between radians and trigonometric identities.
Other
Teachers Choice: How to Convert Degrees to Radians or Radians to Degrees
This site offers a discussion on radian measure. The site offers pictures and explanations to clarify the formulas given. (Note: The free download at the end of the page will not work in archived version of site.)