Here are the useful links to official homepages of most popular CAD packages:
Feel free to explore and to be enchanted.
I've also listed some of my favourite CAD forums on a links below-sometimes you'll find more useful tips and tricks and real solutions to your everyday problems when working with particular CAD package there, in users communities, then in all your product's documentation, help files/tutorials or books.
Here they are:
http://www.coe.org/Collaboration/DiscussionForum/ActiveDiscussions/tabid/210/afgroup/6/Default.aspx - "COE" (formerly the CATIA Operators Exchange-http://www.coe.org/)
http://www.eng-tips.com/threadforum.cfm?lev3=70 - "Eng-Tips" - Forums for engineering professionals (http://www.eng-tips.com/).
Hundreds of technical support forums and mutual help system for engineering professionals.
The two links bellow point to forums in Russian and Serbian that I also like to visit often:
http://www.elitesecurity.org/f161 (General CAD/CAM Serbian forum)
http://www.plm-forum.ru/forum/ (Catia Russian forum)