I love Mailplane. Ever since moving all my email into the cloud (via Gmail), I’ve been learning to love Gmail for all it’s spam-killing and easy consolidation of my more than a dozen email accounts.
BUT, I still remain dubious about the Web OS and browser-based applications. I still find it tough to keep track of my most-used web-based applications. Tabs and bookmarks just don’t cut it. The dock/taskbar and app switcher (Command-tab) is there for a reason, and remains for me to be the anchor on my desktop. If I cannot find an application by hitting command-tab, it is lost to me.
Mailplane turns GMail into an OS X desktop app.
That’s why Site Specific Browsers are such a win for me. Using an application like Fluid, I can create pseudo-applications that are basically a web browser running a single website. They get their own icon and will appear by themselves in the taskbar and the application switcher when running. As you can see here, Gmail (and its 678 unread messages *gulp*) is just a keystroke away.

Mailplane takes the SSB concept to a new level, being designed and tailored just for Gmail. They implement image drag and drop for attachments, integration with the desktop as the default mail application, and little things like allowing me to customize the font size. It truly turns Gmail into a fully integrated desktop application.
I just purchased a license…

… no, thank you.
Check ‘em out:
