Possono essere sempre utili da tenere a portata di click. (via mashable)
Traduco in italiano solo Gmail (appena ho un attimo anche le altre, ma un po’ di inglese so che lo masticate anche voi, vero?).

Gmail

c – componi nuova mail
/ – cursore nel box di ricerca
k – apri una nuova conversazione
j – vai su conversazione precedente
n – messaggio successivo
p – messaggio precedente
o or Enter – apre una conversazione
u – torna all’elenco conversazioni
y – archivia una conversazione
m – mute (archivia e rendi i messaggi futuri di questa conversazione nascosti nella inbox)
x – seleziona una conversazione
s – metti la stellina a un messaggio a ad una conversazione
! – riporta come spam
r – reply ad una mail
a – reply a tutti
f – inoltra message
Esc – esci
ctrl+s – salva come bozza

combinazioni

tab then Enter – invia messaggio
y then o – archivia la conversazione e vai alla successiva.
g then a – vai a tutte le mail
g then s – vai alle conversazioni con la stellina
g then c – vai alla lista contatti.
g then d – vai in bozze
g then i – vai alla inbox

Google Reader

j/k – selects the next/previous item in the list
space/shift-space – moves the page down/up
n/p – in list view, selects the next item without opening it
o – in list view, expands or collapses the selected item
enter – in list view, expands or collapses the selected item
s – stars the selected item
shift-s – shares the selected item
m – switches the read state of the selected item
t – opens the tagging field for the selected item
v – opens the original source for this article in a new window
shift-a – marks all items in the current view as read
1 – displays the subscription as expanded items
2 – displays the subscription as a list of headlines
r – refreshes the unread counts in the navigation
shift-n/p – selects the next/previous subscription or folder in the navigation
shift-x – expand or collapse a folder selected in the navigation
shift-o – opens the item currently selected in the navigation
gh – goes to the Google Reader homepage
ga – goes to the “All items” view
gs – goes to the “Starred items” view
gt – allows you to navigate to a tag by entering the tag name
gu – allows you to navigate to a subscription by entering the subscription name
u – hides and shows the list of subscriptions
? – displays a quick guide to all of Reader’s shortcuts

Google Calendar

calendar view

c – Allows you to create a new event. + c allows you to create an event in a new window.
/ – Places your cursor in the search box
p – Moves your calendar view to the previous date range
n – Moves your calendar view to the next date range
t – Moves you to the current day
d – Displays your calendar in the “Day” view
x – Displays your calendar in a customized view
w – Displays your calendar in the “Week” view
m – Displays your calendar in the “Month” view
a – Displays your calendar in the “Agenda” view
q – Opens “Quick Add”
Esc – Brings you back to the default calendar view

event view

Enter – Expands a collapsed region
Tab – Moves your cursor to the next field
Alt then s – Saves the event
Esc – Brings you back to the default calendar view

Google Maps

arrow keys – pan the map in various directions
page up, page down, home and end – pan out even wider
+/- – zoom in/out

Google Video

space bar – play/pause
right/left arrow skip -jump back 5%
up/down arrow – increase/decrease the volume 5%
m – mute
f – toggle full screen
Esc – return from full screen to normal mode

Google Docs & Spreadsheets

Ctrl+B – bold
Ctrl+C – copy
Ctrl+I – italicize
Ctrl+P – print
Ctrl+S – save
Ctrl+U – underline
Ctrl+V – paste
Ctrl+X – cut
Ctrl+Y – redo
Ctrl+Z – undo
Shift+Tab – move to previous cell/row
Tab – move to next cell/row

Wikipedia

+ – add a new section (talk pages only)
. – opens your user page if logged in
= – protect/unprotect the current page (sysops only)
c – shows the content page associated with the current article
d – delete/undelete the current page (sysops only)
e – edit this page/show source of current page
f – search Wikipedia
h – current page’s history
j – shows all of the pages that link to the current one
k – shows recent changes in pages linked to the current one
l – opens your watchlist (logged – in users only)
m – move the current page and its talk page (non – move – protected pages only)
n – opens your user’s or IP’s talk page
p – shows a preview of your changes (on edit pages)
q – shows a list of all special pages
r – shows a list of recent changes to the Wikipedia
s – saves the changes that you have made (on edit pages)
t – opens the current article’s talk page
u – allows you to upload images or media files
v – shows what changes you made to the text (on edit pages)
w – adds the current page to your watchlist (logged – in users only)
x – loads a random article
y – opens a list of your user’s or IP’s contributions
z – goes to the Main Page

Yahoo! Mail

m – check mail
Shift+m – check all mail
Ctrl+ – close current tab
n – new message
Shift+n – new message in its own window
r – reply
Shift+r – reply in a new window
a – reply all
Shift+a – reply all in a new window
f – forward message
Shift+f – forward in a new window
k – mark as read
Shift+k – mark as unread
l – flag
Shift+l – clear flag
del – delete item
p/Ctrl+p – print
Ctrl+s – save draft
Ctrl+Enter – send message
v – turn reading pane on/off
Ctrl+[ – navigate through tabs
Ctrl+] – navigate through tabs
Enter – open message in its own tab (when message is selected)
Enter – edit contact info (when contact is selected)
Ctrl+f – find a word or phrase in message
F11 – expand window to max height
Ctrl+. – next message (in message tab)
Ctrl+, – previous message (in message tab)
Ctrl+Alt+Shift+up arrow/down arrow – next/previous message
Ctrl+Shift+End – skip to oldest unread message
d – move message to folder
Esc – close read – message tab
Ctrl+Shift+End – start a new chat

WordPress

All letters uppercase – effectively, it means you have to press Alt+Shift+key

Alt+P – publish
Alt+B – bold
Alt+I – italic
Alt+Q – Blockquote
Alt+T – more (Read More tag)
Alt+U – ul (Unordered list)
Alt+O – ol (Ordered list)
Alt+L – li (list item)
Alt+A – link
Alt+C – code
Alt+S – ins
Alt+D – del