Official Class Reference:

http://www.google.com/apis/maps/documentation/reference.html#GPolygon

http://www.google.com/apis/maps/documentation/reference.html#GPolyline 

GPolygon Examples: 

http://econym.googlepages.com/inside.htm

http://www.mailbag.com/users/joe_/clkpoly.htm 

Comparing different techniques while detecting mouseclicks on polygon

HowTo speed up detecting mouseclicks on polygon using boundingboxes

http://freesync.ath.cx/maptest/poly_click.php

This sample is based on :

http://econym.googlepages.com/inside.htm

Create n-sided polygons, circles, or stars (using GPolygon):

http://maps.examples.googlepages.com/gpolygon.htm

Drawing neighborhood border shapes (hard-coded coordinates) and increasing the opacity of GPolygons on mouseover:

http://xmaps.busmonster.com/poly.html

Diagrams with GPolygon/GPolyline. Clock as an example of dynamic pie.

http://koti.mbnet.fi/ojalesa/exam/polytime.htm

GPolyline Examples: 

Getting your polyline to work in IE (important!):

http://www.google.com/apis/maps/documentation/#XHTML_and_VML

Encoding a polyline with 3 points (links to polyline creating utility,  compression algorithm explanation): 

http://www.google.com/apis/maps/documentation/#Encoded_Polylines

Create and drag a circle radiating from a particular point (using GPolyline):

http://maps.huge.info/dragcircle2.htm

http://maps.huge.info/dragcircle.htm (simple version)

http://econym.googlepages.com/basic7.htm 

Drawing GPolylines with arrowheads:

http://econym.googlepages.com/arrows.htm 

Simulating "scratched" letters on the map with GPolyline (defining custom shapes):
http://gmaps.examples.googlepages.com/gpolyletters.htm?message=WE%20%3C3%20GMAPS&lat=38.54816542304656&lng=-162.421875&zoom=3