OS X Mail client alternative Sparrow just launched version 1.4, and it adds a unique approach to attachments via Cloudapp integration. Instead of attaching a large file to your message, you can opt for a CloudApp link; drag your large file into the body of your new message, and the file automatically uploads to CloudApp's servers and a link gets dropped into your email. For the attachment traditionalists among you, drag the file to the bottom of the message window (or hold down command while dragging).
Aside from the usual batch of UI and speed tweaks, 1.4 also adds iOS-style 'pull-to-refresh' for manually grabbing new emails, and Gmail-style conversations where the oldest message is shown at the top of the thread (enable in Preferences). Grab the free update in the App Store. The new Growl 1.3 recently mangled Sparrow support, but the Sparrow team promises 1.4.1 should fix your notifications.