Firefox/Internet Explorer: If Gmail or Google Apps mail is attached to your workday like your right arm, MailBrowser could be a handy pop-out sidebar tool for making sense of the people who mail you and the files they send.
"Could be", in this case, because MailBrowser, which is basically a Firefox or Internet Explorer sidebar that's aware of what you're looking at in Gmail or Google Apps, is somewhat of a mixed bag, with the total payout depending on how you use Gmail. If having a complete backup of your IMAP history cached locally for easy message access sounds appealing, your sidebar is going to hold a lot of helpful data. You get a complete list of everyone you've ever contacted, from one or more Gmail/Apps accounts, and you can tag contacts to make them friendly to searching and sorting.
As for file attachments, MailBrowser is even more helpful on that front. Because your files are cached and stored locally, you can use any desktop search software, or just Windows' own file indexing, to find the stuff you've been sent. MailBrowser also provides thumbnail previews of the files you're digging through, so it's easy to pin down the Word document you're looking for.
Then again, MailBrowser takes a seriously long time to download the average account, as you might expect, and isn't always on point when it comes to handling multiple accounts, multi-person emails, or even people who have emailed more than one account you've hooked up.
If having your attachments and contact information at your fingertips, and locally cached, is a valuable resource for your Google-Mail-addicted ways, you could do worse than MailBrowser. It's a free download for Windows systems only, and requires Firefox 3.0 or later or Internet Explorer 7 or 8.