Here are few useful web apps that will enhance your mailing experience, you will be amazed by the functionality of these add-ons.

AwayFind.com
If you are busy or on a vacation but still have to check emails just because you can't afford to miss some “urgent” messages from some important source. Then use AwayFind, you can easily create alerts for important emails. You are notified via phone calls or text messages, without having to constantly check your mailbox.
Boomerang
If you need scheduled sending in Gmail i.e. you want to compose an email now and send at a later time. Then use this service, with this you can easily schedule birthday notes when you have time to write, and Boomerang will dispatch them at the right time.
FindBigmail.com
TripIt
For TripIt users this Gmail add on will check your inbox frequently in a day for any travel plans and will imports travel plans to the TripIt account automatically. So that you will not be needed to manually forward emails to TripIt account. If there are already trips with matching dates, TripIt will add plan to your trip else it will create new trip.

AwayFind.com
If you are busy or on a vacation but still have to check emails just because you can't afford to miss some “urgent” messages from some important source. Then use AwayFind, you can easily create alerts for important emails. You are notified via phone calls or text messages, without having to constantly check your mailbox.
Boomerang
If you need scheduled sending in Gmail i.e. you want to compose an email now and send at a later time. Then use this service, with this you can easily schedule birthday notes when you have time to write, and Boomerang will dispatch them at the right time.
FindBigmail.com
This service only works with gmail but is an awesome tool. This can be very helpful for you in case the provided memory on Gmail in fall short for you.This service can help you to find largest emails in your inbox and sent item and delete or move them to free some space. This application uses IMAP and gets the sizes and not the content of email . It uses a temporary access token, this app won’t have access to your Gmail account afterwords, this feature makes this app safe to use.
This is a very simple app that cleans your Inbox and makes your mailing efficient. Other Inbox scan your mailbox for unimportant mails and sorts them into relevant directories. For instance, news alerts from some news site would go into News folder and those from sports site will go to sports folder. This app works with Yahoo, Gmail, AOL and some other sites.
This is one of my favorite app. If you get email from an unfamiliar person, and you want to know about this person then Rapportive will bring all that information about that person right inside your inbox. Rapportive searches social sites like LinkedIn, Facebook, Twitter, etc for an email sender using the email address. And shows you the detail of that particular person.
For TripIt users this Gmail add on will check your inbox frequently in a day for any travel plans and will imports travel plans to the TripIt account automatically. So that you will not be needed to manually forward emails to TripIt account. If there are already trips with matching dates, TripIt will add plan to your trip else it will create new trip.
This app adds a “unsubscribe” button to your Inbox so you can conveniently unsubscribe from mailing lists and newsletters just with one click. This app stops sender from sending messages to your inbox. It works will all major mailing websites.
Additional resources for Gmail
If you are using Gmail then check Gmail Labs. You will find huge collection of features like chat box, translator for emails and other tools developed by Gmail team. You can also use add-ons for your gmail from Firefox and Chrome app stores.

