Many Mailplane users asked for Google Calendar integration. Now Google released new Google Lab features to embed calender and documents in the Gmail sidebar. The latest Mailplane 2.0 release supports these two features.
From the Gmail Blog:
To get you started, we've worked with the engineers from the Calendar and Docs teams on two highly requested features: a simple way to see your Google Calendar agenda and get an alert when you have a meeting, and a gadget that shows a list of your recently accessed Google Docs and lets you search across all of your documents right from within Gmail.
Tip: To customize your sidebar, try the Navbar drag and drop lab feature.
Check the Gmail Labs group for support and feedback.