This is an exhaustive post to bring to notice all the terrific features of Gmail and Google Apps Mail. Though this might not go into detail to all of the existing features, it covers those which i feel should be tried by everyone, if ignored or unknown, to increase their own productivity and management. Here they come
Labels :
Entry of Gmail with revolutionary mail storage space has created a ripple effect to its competitors to increase their mail space drastically and some made it unlimited even. I feel this changed the stereo type services provided until then. Apart from mail storage another revolutionary feature was Labels. Many wonder why gmail doesnt provide folders but labels instead, and even i saw many happy without folders nor even using the provided labels. In plain english labels can be termed as tags, With this we can infer that a single mail can be tagged with different tags. In the case of folders, a mail can be moved into only one folder which might not be suitable in many occassions. For instance take my case of a mail from a family member, I can Label it as Family, Folder does the same until here, but if the mail contains photos, i also need to add Photos Label, so that it can be found in both

Family and Photos. But with folders, one mail to one folder. With further addition of color coding to labels, it becomes dead easy to identify our mails.
Filters :
Suppose if you want to mark mail from your boss which has words, immediate and urgent, with star and label it with name Important, you could do this within no time with filters. Go to Settings > Filters > click on Create New Filter
so, if you get a mail with the above criteria, you have no mail management work to do and can immediately concentrate on content.
Labs :
Labs are experimental features which can be enabled by the user, the user can select any number of such functionalities which suits him best. There are many such additive functionalities like offline access, Tasks, etc., there are also minor tweaks such as quick links, mouse gestures, custom keyboard shortcuts to name as few.
My feeling is that every one should try all the lab features, some are time saving, productive and even funny. Just give it a try in leisure.
Tasks :
This is a very very simple feature which can be enabled from Labs. (the above feature). Tasks list can be created and tasks can be added into the list. Once done tasks can be strikeout to mark that as complete.

Fetch POP mail:
With all the great features, Its very much beneficial if we could check our mails other than Gmail’s in its interface. Fetch POP mail is a boon for such requirement. I haven’t heard of Fetch POP in any web interface prior to the entry of Gmail. With such rich functionality Fetching other mails and Filtering them with a label will be deadly. So no need of checking mails with numerous logins in different site.
Send Mail as :
We could receive other mails in POP, fine! What if we need to send mail from the POP received mail. Gmail does address this issue as well. Not necessary to login to different sites to send from another ID. Just a quick authentication to send mail from other id to check your ownership of the ID.
While composing a mail, select a mail id from which you want to mail to. That's it.
Keyboard Shortcuts :
This is inarguably the one of the best features of Gmail. Hit "?” in your mail, here comes all the existing keyboard shortcuts. Pretty simple, cool, rich and whatever, this feature makes the application indifferent to any desktop software. Further custom shortcuts can also me made by enabling a labs feature.
Spell check :
Spell checking in Gmail is great. But what makes it exemplar is that it could spell check in over 36 languages as of now. Sure that more will be added into this list. After composing mail hit on Check Spelling link and Gmail comes to your rescue and highlight any grammatical blunders you have made.
Enlightened ?, wondering ?, excited ?, Yes if you aren’t aware of any of these features, try to use them and you will say that Gmail is awesome. One best thing i found in Gmail, for that matter any of the Google’s product, is that they continuously improvise and innovate. No saturation point for innovation isn’t it ? Thinking from the end user side is what makes Gmail success exceptional from any other webmail's. Rock on Gmail.