Linux only: The Evolution app that's loaded with most Linux systems is a heavy thing. If you'd just like to keep tabs on your inbox and write emails, Postler is a great alternative email client.
If you're loading Postler with a Yahoo or Gmail account, Postler sets up with just an address and password. It imports your folders, starts grabbing your mail, and does it in pane that's remarkably uncrowded. On Ubuntu, Postler also works through your main messaging window, so you can receive shadowed notifications of messages.
Postler is a work in progress. It's nice to be able to hide folders, but for a client so otherwise friendly to Gmail, it would be handy to have it thread conversations together. But it's a nice, low-noise alternative of a desktop/offline email client — kind of akin to Sparrow for Mac. Postler is free to download for Ubuntu; to install it in Ubuntu, run these lines in your Terminal:
sudo apt-add-ppa repository: postler-dev/ppa
sudo apt-get update
sudo apt-get install Postler