Dan Warne has written a very nice article about "cloud computing"...
I’ve finally decided to rid my e-mail and calendar of this earthly life — I decided after a recent MacBook Pro upgrade that it was time for them to go and live in the cloud.
...he mentions Mailplane as one of the apps that let him do the transition...
MailPlane is a stupendously fantastic program — it recreates an e-mail client around Gmail.
...and he explains the difference between Mailplane and other mail applications:
“Sure”, you might say, “I can already get my Gmail in Apple Mail through POP or IMAP.” The difference is MailPlane is a true web app, in that all the data resides on Google’s servers and MailPlane provides (as the author says) “the last mile” between the web app and your desktop. MailPlane adds back the features you’ve come to rely on your desktop mail client for, like Growl alerts when you get a new e-mail, the ability to drag a file to its icon and have a new e-mail open up with the file attached. Or the ability to select a batch of photos in iPhoto and click “e-mail”.
It's a great read, check out the full article.