Some random and useful things to take advantage of in Gmail:
Vacation Auto Responder. If you know you're going to be away
from Gmail, use this tool (found in Settings under the General tab) to
let people know the details when they drop you a note.
Mute Conversations. Conversation threads in Gmail are handy,
but you don't want to be part of every conversation. When reading such
a message hit the M key. This archives the conversation and all
subsequent replies as they come in. You can still find them with a
search, if you change your mind.
PDF, TIFF, and PPT Reading. You can read a number of document
types on handhelds by accessing them through Gmail. The same is true
with Gmail on the desktop when it comes to PDF files, TIFF images, and
PowerPoint presentations. You don't need Adobe Reader or FoxIt Reader
installed, nor that infamous Microsoft Office app. Access any message
with one of those document types attached, then click the View link
next to the file size. It will open the file in your Google Docs
account.
E-mail to Document. This is very handy for a particularly
large conversation in Gmail, but works with any message. In Gmail Labs
(click the green beaker icon), enable "Create a Document." This adds a
link to the collection at the right of your message named, surprisingly
enough, "Create a Document." Click it and Google Docs loads the thread
into your default word processing document.
Secure Gmail. If you're concerned about snoops and want the equivalent of PGP for Gmail, get the FireGPG extension
for Firefox. It activates public-key cryptography for securing your
messages in Gmail using Gnu Privacy Guard, an open-source
implementation of the OpenPGP standard.
Suggested Recipients. The very latest in Gmail Labs
excellence, this experiment suggests recipients you might want to
include in the To: field, based on the content of the message and who
you've e-mailed before.
Insert Images. It's possible to stick a picture into your
messages in Gmail, not as an attachment, but so that others will see
the image upon receipt (fellow Gmail users will still have to click
"Display images below"—that's part of Gmail's security). Turn the
service on in Gmail Labs, then look for the picture icon in the toolbar
when composing a message in rich formatting mode.
Delete Spam Now. Visit the Spam folder occasionally. Gmail
has what is probably the best spam blocker in the world, but
occasionally it can be overzealous and categorize your friend or a
newsletter you want as spam. If this has happened, mark the messages
you want to save as "not spam" and then click the Delete Spam Now link
to clear the rest.
Multiple Signatures. Gmail has built in only one signature
for use on all messages you send. Several Firefox extensions will
provide multiple signatures from which you can pick and choose,
including Clippings and Blank Canvas Gmail Signatures.