If you are a power user of Gmail you will want to at least give MailBrowser a try (http://mailbrowser.com). It is mostly a straight up clone of Xnobi, an excellent plugin for Outlook, that works in your browser with Gmail. It installs as a regular application and runs in your system try. On Firefox it also installs an addon called MailBrowser that I presume creates the sidebar MailBrowser runs in.
Once you’ve installed the program and given it your login credentials for your Gmail account, it downloads your contacts and all the attachments in your Gmail account. The attachments are stored in a folder on your local machine by contact name, and are indexed by MailBrowser so you can search them. The MB interface presents you with information about your contacts, and what they have sent you either by thread or as a list of files.
Among the helpful things you can do is edit files opened from the MB interface. The changed file is synced back to your Gmail account and you can get a link to email the changes back.
This addon is still under heavy development but has lots of potential. I would like to see it run in Gmail as a loadable gadget to give even deeper integration. But even as is it is very usable.
As far as security goes, the developers say this addon only communicates with Gmail, nothing goes back to them. Your files are stored and indexed on your machine and under your control.