ましょう

Etymology 1
Volitional form of the polite suffix.

Suffix

 * 1)  let's; shall we
 * 2)  I will
 * 3)   probably
 * 1)   probably

Usage notes
This can be considered as the polite form of the volitional suffix ~, except that it is more polite and attaches to the masu stem of verbs.