Compass and Straightedge Constructions with videos: http://www.regentsprep.org/Regents/math/geometry/GC1/indexGC1.htm
Basic Constructions: http://constructions.homestead.com/
Triangle centers:
Orthocenter - Altitudes ( vertex perpendicular to opposite side)
Circumcenter - Perpendicular bisectors of sides - a circle with the circumcenter as its center will circumscribe the triangle
Incenter - Angle bisectors - a circle with the incenter as its center will be inscribed in the circle
Centroid - Medians (vertex to midpoint of opposite side)
The Euler Line - orthocenter, centroid, circumcenter, (also incenter when isosceles)