Windows/Mac with Firefox or Chrome: The CloudMagic browser extension adds an incredibly fast and effective as-you-type search box to your Gmail account capable of searching across multiple accounts in one shot and previewing emails without leaving what you're doing.
First, a note on privacy: CloudMagic stores your password and data locally, on your computer, so you don't have to feel too creepy about handing a third party the keys to your Google account. Once you install the extension and add your account information, CloudMagic starts indexing your inbox. Indexing time will vary depending on how big your account is, but recent email will be almost instantly searchable. You can narrow your search using many of the same operators available in Gmail, and, as you can see in the video above, the results are fast and the inline previews are particularly handy when you're composing an email and don't want to leave the window just to look up a little information. In my experience, Gmail search speeds slow down significantly the more email you receive, and so far CloudMagic's lightning fast search is feeling like a godsend.
CloudMagic is a free plug-in for Windows and Mac, works with Firefox or Chrome; it supports regular Gmail and Google Apps accounts. If you give it a go, let's hear how it worked for you in the comments.