Keyxl.com is the self-proclaimed largest online database of keyboard shortcuts in the world. The database is searchable by software program and covers all the major operating systems including Windows, Macintosh, Linux and browser-based applications.

I use keyboard shortcuts all

time and find them useful and time saving.

Some of my favorites:

Windows:
Windows+D
Alt+F4
Ctrl+A
Ctrl+C
Ctrl+V
Alt+Tab
Ctrl+F

Opera browser:
Z
X
1, 2
F11
Ctrl+F
F2

Google Reader:
Shift+N
Shift+O
Shift+A
1, 2

Image source: Openclipart.org, public domain.

Original post by Clinical Cases