Software and Sites for investigative work ??

What is some of the software and web sites that are used to do investigative work regarding emails (origination, etc), digital photographs (authentication, whether they have been edited, etc)? Essentially, the more basic and free the better. I'd also like to know whether a photo has been downloaded from facebook, twitter or some other site.

I use google images to search for similar images when I'm trying doing some investigative work. I have some basic understanding of EXIF info as well. For emails I know some basic info about trying to determine the originating IP address in the email header and do more searching to see who that IP is assigned to.

Are there some favorite tools you like to use?

 

List of digital forensics tools:
-http://en.wikipedia.org/wiki/List_of_digital_forensics_tools

List of over 115 free tools for digital forensics analysis:
-http://forensiccontrol.com/resources/free-software/

 

thanks, I'll check these out