You can add new features to your Gmail mailbox in multiple ways.

There’s the official Gmail Labs section where you’ll find loads of “experimental” features developed by members of the Gmail team itself. Need to translate an email message ? Or want to move the Gmail chat window to the right? You can do it all with Gmail Labs.

You then have contextual gadgets that are triggered based on the email content. For instance, if an email has a link to a YouTube video, you’ll be able to watch that video inside Gmail itself with the magic of gadgets. There’re also Sidebar gadgets that can bring external content like news, maps, to-do lists, etc. into Gmail.

Then there’s an entire army of Gmail add-ons for Firefox and Google Chrome. Whether you want to manage multiple Gmail inboxes or need a stunning signature for your outgoing email messages, add-ons will have you covered.

Finally, you have web apps for Gmail that, like extensions, are also created by third-parties but the big difference is that these apps mostly interact with your mailbox through Gmail APIs and are a lot more powerful. For instance, you can’t expect extensions to sort your Gmail messages by size but a web app will be able do that.

The Best Web-Apps for Gmail

Let’s now look at some useful web apps that will help improve your Gmail experience. They should also work with your Google Apps for Gmail accounts.

1. Other Inbox – This is perhaps the easiest way to cleanup your Gmail Inbox.

Other Inbox scan your mailbox for unimportant emails and automatically sorts them into relevant folders. For instance, news alerts from CNN would go into the News folder while emails from Amazon or iTunes will be labeled as Shopping.

2. Unsubscribe.me – This adds a little “unsubscribe” button to your Gmail Inbox allowing you to unsubscribe from mailing lists and unwanted newsletters with the click of a button.

It won’t move any future messages from that sender to the trash, it just gets the sender to stop sending you any more emails.

3. FindBigmail.com – When you are running out of space on Gmail, use this service to find the largest emails in your account and delete /download them to recover some space.

FindBigMail connects to Gmail via IMAP and retrieves only the sizes and not the full content of the email messages. It uses a temporary token and therefore won’t have access to your Gmail account forever.

4. Rapportive.com – If you get an email from an unfamiliar person, you don’t have to use Google to know more about him or her as Rapportive will bring all that information right inside your Gmail inbox.

Rapportive takes the email address of the sender and builds a profile based on all the other social networks – like LinkedIn, Facebook, Twitter, etc. - where that person may have a presence.

5. AwayFind.com – You are on a vacation but still have to check emails just because you don’t want to miss any of the “urgent” messages from your important clients?

With AwayFind, you can easily create alerts for important emails and get notified via phone calls or text messages without having to constantly check your mailbox.

Related: Get read receipts for Gmail

6. Boomerang – With Boomerang for Gmail, you can compose an email now and schedule it to be sent a later time. Thus you can easily schedule birthday notes when you have time to write them and have Boomerang dispatch them at the right time.

7. Hotmail.com – How do you access your older emails when the main Gmail service is down and you don’t have an offline backup?

It is therefore recommended that you use a non-Gmail service to backup your Gmail messages and Hotmail perfectly fits the bill here. This archive should also come handy incase your Gmail gets hacked.

8. TripIt – The service will scan your email inbox multiple times a day for any travel plans and automatically imports them to your TripIt itineraries without you having to forward any email message to TripIt.

If you already have an existing trip with overlapping dates, TripIt will add the plan to your trip else it will create a new trip.

PS:If you are a Microsoft Office user, check a similar a list for Outlook Add-ons.

[**] This article was updated to remove references to Email Oracle and Etacts as both these services are no longer available.