Gmail Labs lets you add two gadgets in the left sidebar: Google Calendar and Google Docs, so you can add events, see your agenda and open recently-edited documents. There's also an experimental feature that lets you add any gadget by entering the address of its XML file, but I couldn't find gadgets that work and look well.
"We realize this isn't very user friendly right now; it's a sandbox mainly aimed at developers who want to play around with gadgets in Gmail. We're not tied to the left-nav as a primary way to extend Gmail -- in fact we think it is relatively limited and doesn't offer scalable real estate. There are also some downsides to the iframe-style Gadgets we're using today -- they can sometimes slow down the page. We're fanatical about speed, so we'll be keeping a close eye on performance," says Dan Pupius.
To make room for the gadgets, you could try these experimental features from Gmail Labs: Right-side chat, Right-side labels (move Gmail Chat and the list of labels in a right sidebar) and Navbar drag and drop (reorder containers).
{ Thanks, roody. }