Wolfram Research
Wolfram Math World: Perpendicular Bisector
MathWorld's definition of a perpendicular bisector, with illustrations to show the different uses of the perpendicular bisector.
Texas Instruments
Texas Instruments: Perpendicular Bisector of a Line Segment
Students learn to draw a segment and its perpendicular bisector. They understand right angles and congruent segments. Students observe that changing the length or orientation of the segment changes the perpendicular bisector.
BBC
Bbc: Ks3 Bitesize: Constructions and Locus
This BBC Bitesize focuses on constructions and locus. To understand this activity, you must already know how to define perpendicular, bisect, and locus.