A site-specific browser (SSB) is a great way to "package" a web app you use every day into a dockable, clickable app that maintains its own cookies, settings and preferences versus your everyday web browser config. On the Mac, the easiest way to make an SSB is via the handy Fluid app, which supports independent prefs in its paid version.
Chrome's preliminary "packaged app" support, which will take the SSB concept to the next level, has been supported via dev builds on Windows and Linux since the beginning of the month; you can get a sneak preview of the App Launcher on the Mac, but actual packaged app support is still TBD.
If you love the Gmail-savvy simplicity of Mailplane, but can't quite get around the price tag, you might consider an SSB for Gmail, which gives you some of the same functionality. Unfortunately, Fluid builds its SSB support on top of Safari and Webkit, which means your SSB won't be quite as Gmail/Google Docs-savvy as it would be if you were using Chrome. (Try printing a font-heavy Google Docs file from Safari.)
There is a way out of this pickle: build an SSB using Chrome as the underlying engine instead. With a quick script and a few Terminal tweaks, you can make a double-clickable fresh Chrome SSB that keeps its own profile well clear of your normal settings. Don't like shell scripts? The CreateGCApp utility packages up the script and does all the work for you.
Having a separate instance/SSB of Chrome is particularly handy if you have trouble with Gmail's multiple-account support in your regular browser, as sometimes can happen if one account uses single sign-on via a third party.