Software:

Adobe Systems, Inc. The developers of Photoshop, Illustrator and more!
eFrontier Developers of 3D figure modeling application Poser
Alien Skin EyeCandy and Splat plug-ins
Flaming Pear Flood, Lunar Cell and many other plug-ins
AV Bros. Page Curl plug-in

Commercial image libraries:

thefullmontage Image library specifically geared towards photomontage artists
AbleStock Huge collection of traditional photography and cutout objects
Stockbyte Traditional image library, with many collections of cutout images
AbsolutVision Cut price collection of both traditional and cutout images
Photodisc One of the largest traditional photo libraries
PhotoObjects.net Vast and varied collection of people and objects with clipping paths
Photos.com Selection of cutout images of people and objects
iStockPhoto Very low cost and huge collection of images of all types

Free image libraries:

Stock.Xchng Free image exchange, photographs in many categories
MorgueFile Free image resource mainly provided by two photographers

Other free image resources:

Defense Link Public domain images from the US Department of Defense
NASA Public domain images from everyone's favorite space agency
Dryden Research Many photographs of aircraft, in the public domain
NOAA Photo Library More public domain images of weather than you can handle