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
Related:
The Best Keyboard Shortcut in Firefox - Type Web Addresses Quickly. Digital Inspiration.
Netflix Movie Player Keyboard Shortcuts http://goo.gl/uoyZ
G Docs Spreadsheets shortcuts http://goo.gl/NTnCp
Image source: Openclipart.org, public domain.