Gmail has a bunch of lesser-known feature that can end up being very useful once you get to know them. I've put together a list of my top 10 favorite features that you may or may not be familiar with. Here are the first five; watch for the second batch sometime next week.
10. Custom "from": Most people end up managing more than one email account -- some are personal, others might be for work or school. When I graduated from college, I wanted to keep my .edu address for alumni-related things, but made Gmail my primary personal address. My university made it easy to forward my .edu mail to my Gmail address, so I could read all my mail from my within my Gmail account. I was happy to find out that Gmail would actually let me send mail "as" my .edu address, so I could continue to keep that identity, while managing all my email from one place. Here's how to set that up.
9. Open attachments in Google Docs & Spreadsheets, or view as slideshow: If you are sending a Word document or Excel file as an attachment, Gmail lets you easily open it in Google Docs & Spreadsheets. (In case you're not familiar with Google Docs & Spreadsheets, it's our online word processor and spreadsheet application, which lets you store and access documents online and collaborate on them with anyone.) Since other people are working on many of the docs I receive as attachments collaboratively, it's really easy to just open them in Docs & Spreadsheets and create a single document to work from, rather than constantly sending versions of documents back and forth. If the attachment is a PowerPoint presentation, Gmail will recognize this and give you a link to"View as a slideshow." This will open a window with a Flash preview of the slides. This is great for quickly reviewing slides in the browser.
8. Gmail gadget for iGoogle: I use iGoogle to bring together a lot of the information I care about on the web (feeds, my Google Calendar, YouTube videos, etc). I use it as a dashboard at the start of my day to get an overview of what's going on in the world. I've added the Gmail gadget to my page to get a preview of my Inbox, which is great because it's one of the first things I'm interested in seeing.
7. Reply by chat: Most people know that you can chat with your contacts in Gmail if they're using Gmail too. But there are some subtle features that make chat particularly useful in the context of using your email, like replying by chat. If you've received an email and notice that the sender is online (by seeing the little green dot next to his or her name in the conversation), you can click the button "reply by chat." What's particularly convenient about this is that the chat will be archived as part of the email thread to which you replied. I like this most because it means when I search for anything related to that thread, I find the chat transcript alongside all the relevant email messages.
6. Gmail for mobile application: It wasn't that long ago that my cellphone could barely handle sending text messages. I didn't used to think of it as being an efficient place to access my email. That's changed -- big time. Email has become one of my primary forms of communication when on the go. Unfortunately, on a lot of phones, dealing with email can still be a pretty annoying experience, especially if you're not using a device like a PDA. Enter Gmail for mobile. It's fast, it syncs with your online account, and it gives you virtually all of the same Gmail functionality like search, filters, and access to your whole archive of mail.
Source:→ Gmail Blog