Simple only if you are a keyboard person :)
A prerequisite for the tips to work is to enable the keyboard shortcuts in the gmail page.
1. "gi" - When you are in any part of the gmail interface, reading a mail, spam box, drafts etc, you dont have to move your hand till the mouse.. and then move the mouse to the inbox link.. just type the letters "g" and "i" in succession and you will be taken to the "inbox"
2. "ga" - This option is similar to that of "gi" but this shortcut takes you to the "All" box, i.e It will take you to a place where you have all your mails, chat conversations and muted mails (more on muted mails later)
3. "c" - takes you to the Compose mail option :)
4. "Esc" you are in compose mail mode, you want to discard the current composition and get back to the inbox, when you type "gi" it is being typed in the address box or the textbox... What to do ;) for.. most of you who are familiar with VI editor, you know what to do. Just type "Esc" button and you are in to the command mode, then just type "gi" "ga" etc to go to the view that you want :)
5. "r" to reply to a mail, when you are reading some mail.
6. "a" to reply to all, while reading a mail
7. "f" to forward... i didn't have to tell that .. isn't it :)
8. "m" - You are reading a mail thread. ex. Friends gossiping on a particular topic.. or you are being dragged in to an unnecessary conversation.. you dont want to look at all those mails. what do you do.. you just "mute" it. Once you mute, those mail conversations will not appear in your inbox, if you want to view all the conversations including the muted mails, you just have to do "ga" and you have your conversations.
9. "z" - this option will undo the previous action that you have taken. For Example you have deleted a particular mail, and you want to undo that, just type "z"
10. "/" the best feature that i have found on gmail ;). Typing "/" will focus the cursor on to the search text box and from there starts the following host of tips that will make your life easier ;)
11. "/is:<label>" - typing "/" will take you to the search box, then you type "is:" followed by any of the labels that you have applied or the mail already has. Few of the default labels that you can use right away are "inbox" "spam" "chat".
I thought of writing only a few useful tips, but couldn't resist from writing so many.. But this is not the end of the list, i will give a couple of them more and let you guys to explore further.
13. "*a" will select all the conversations, "*n" will unselect the selected conversations. "*u" selects all the unread conversations etc etc
14. The most important shortcut is the "?" this will give you onscreen help :P
The bestest best feature of gmail, is the usage of the mail aliases. I know that this trick is very old, but will give here for the rest of the crowd who are not aware of it.
You can append your email address, with a "+" symbol and create unlimited aliases. For example I have joined a mailing list called "runners for life". I can submit my mailing address to those guys as "email@example.com" where "firstname.lastname@example.org" is my actual mailing address.
Now create a filter in you mail options, add a new label called "rfl". The rule can set the label when ever the "to address" has "email@example.com". Above all, you can now add colors to your labels ;) the colors are limited in number, but yes they are very helpful. you can search for all the conversations with this label as "/is:rfl" and you have all the mails from the mailing list :)
The advantage with the above trick is that, you can know which of the mailing list or forum has leaked your email address to the spammers :)
I have written lot of stuff, I will stop here as my movie download has completed :)
Will come back with more information :P