I used to think that Gmail's built-in search feature was fast, but that was before I tried out CloudMagic (free). This browser extension (available for Google Chrome and Mozilla Firefox) can search your Gmail so quickly that you'll never be able to look at Gmail's own search tool the same way again.
CloudMagic installs within seconds, and once it's up and running, it appears as a small search bar on the upper right hand side of your Gmail window. Right now, CloudMagic, which is still in beta, works only with Gmail and Google Apps e-mail, but the maker says support for Google Docs e-mail is coming soon, as is support for Hotmail, Yahoo Mail, and others.
The CloudMagic search bar appears automatically in your Gmail window, but you still need to manually add your Gmail account (or accounts--CloudMagic allows you to add as many e-mail accounts as you'd like, and you can search the contents of one account from within another) by offering up your user name and password. CloudMagic says your password is stored locally on your own PC, similar to the way an e-mail client such as Thunderbird would store it, so you don't have to worry about its security.
Once you've added your accounts to CloudMagic, it begins indexing all of your messages. The initial indexing process takes some time (more than an hour on my Gmail account, which currently holds more than 5,000 messages), but you can begin using CloudMagic before the indexing process is complete. I was surprised at CloudMagic's retrieval speed even during ongoing indexing.
CloudMagic displays search results as you type, and the results appear in lightning speed. The list of results includes contacts and messages, and appears in a panel that drops down from beneath the CloudMagic bar. You can see the sender, subject line, and a brief preview of an e-mail message; clicking the message brings up another panel with its full contents, which appears to the left of the CloudMagic results list. Rather than crowding your screen, CloudMagic's layout actually makes it easy to preview a message that you're searching for, while still allowing you to view your Gmail unobstructed. This means you can search for one message while composing another, and can copy and paste info from one to the other.
My only complaint about CloudMagic is that each installation works on only one computer; despite the name, your index of messages is not saved to a cloud, where it would allow you to access it whenever you access your Gmail. If you want to use CloudMagic on another computer, you'll have to install it there, too; while the installation and initial index is easy enough to set up, I just wish you didn't have to go through it twice. I typically access my Gmail account on a variety of desktop and laptop computers, and I've gotten so used to CloudMagic's speedy search that I'd like it to be more readily available wherever I go.