This has proven very helpful so far....

http://www.fileinfo.net/

FileInfo.net contains a searchable database of file extensions with detailed explanations of each file type. Every file extension entry contains information about the file format, a description of the file, and how to open the file. Programs for opening the files are listed for both the Macintosh and Windows platforms. For more information about file extensions, view the "What is a File Extension?" page. If you have other file-related questions, please visit the FileInfo.net Help section
Text Files
Data Files
Image Files
Audio Files
Video Files
Web Files
Font Files
Plugin Files
System Files
Executable Files
Compressed Files
Developer Files
Backup Files
Disk Files
Game Files
Misc Files