Gmail is part of Google Apps, the Internet giant's software package that includes e-mail, calendar, documents, spreadsheets and sites (a wiki). Anyone can get a free Google Apps account by singing up for Gmail, while businesses can buy a premier edition for $50 per user per year.

You can install the following three add-ons with either version (free or paid) of Gmail. To add them, you must first go to Gmail Labs. To access Gmail Labs, click on the green beaker in the upper right hand corner of your Gmail inbox (it appears next to the "settings" tab).

If you like these add-ons, be sure to scan our previous Gmail Add-on reviews: five favorite Gmail add-ons and five (more ) favorite Gmail add-ons . As the Google guys will tell you, all of these features are experimental, so don't be surprised if you see some kinks.

1. Offline Gmail

How it helps do no evil: Call it the airplane problem. One of the hang-ups businesses have had in adopting Gmail for their workers has been offline e-mail access. With traditional, client-based e-mail, it's easy to launch your Lotus Notes or Microsoft Outlook and type out some e-mails while in an area sans Wi-Fi. After balking at the need for offline e-mail for quite some time, as any offline software essentially runs counter to Google's business model, the company finally relented with offline Gmail that allows you to work on e-mails in your browser while you're not connected to the Internet.

With offline Gmail, you can write and read e-mails. Messages that you compose while offline will be stored in your outbox and automatically sent next time you connect to the Internet.

How to set it up: You must first install Google Gears on your web-browser. Gears, a Google-led open source project, allows you to utilize many Web-based applications in an offline mode. After you install it, you probably will have to restart your browser.

Log into Gmail and click on the green Labs beaker. Scroll to "Offline." Click "enable" and "save changes." After you do that, an "offline" tab will appear next to the Google labs beaker and "settings" tab in the upper right corner of your inbox. From here, it's simple. Click on the "offline" tab and it will create a locally stored cache of your e-mail messages (if you have a pretty healthy sized inbox, it will go back about six months).

Doing no good: It would be great if you could control the amount of messages you choose to take offline with greater granularity. I appreciate the fact that the tool doesn't try to download all of the e-mails if you have a big account, but I'd prefer to just take a select 20 or so offline rather than clog up my machine with several months' worth of messages.

Google has listed some other limitations to offline Gmail, including not being able to send e-mail with attachments. This could be a big limitation for some of you, so beware.

2. Gmail Inboxes

How it helps do no evil: This tool basically adds more inboxes, or window panes, to your main Gmail inbox. These boxes can contain e-mails related to frequent search terms that you use. You can also match them to labels that you create to help organize your Gmail messages. For instance, if you have a label for "Travel," you can create a pane that lists your travel messages. This add-on can be helpful if you're hoping to have a front-and-center visual of certain e-mails and don't want to have to keep searching for them.

How to set it up: Go to Labs, click "enable" and "save changes." After your inbox reloads, click on "settings" in the upper right hand corner of Gmail. In the settings menu, you'll see a new "Multiple Inboxes" tab. Click on it. This will let you set what you want to appear in your extra panes (up to four of them). If you type a term into one of the fields on its own, the tool will create a pane with that search term. To add a pane that contains a label, add "Label:" before you type term into the field.