Build one side of a rig, then mirror it.
Tools
In the Scripts menu:
- Mirror selected bones: flips the setup position and rotation of the selected bones across X = 0.
- Swap left/right in names: renames the selected bones (or every bone, when none is selected)
to the other side:
arm_l↔arm_r,hand.L↔hand.R,leftEye↔rightEye. A name that is already taken is left alone.
Each tool is one undo step.