Found some websites with lot of cool transition effects for flex. Right click to view the source and get it for use in your own applications.
Transition effects :
http://www.efflex.org/category/orgefflexviewstackeffects/
Zoom in & zoom out effects :
http://doloresjoya.com/flex/animation/cloudscape/Cloudscape.html
Bounce, expand & collapse effects :
http://doloresjoya.com/flex/animation/bounce/bin-release/Animation.html