Gmail with the text-message authentication; I have to get into my email from too many different machines for there to be a point to using a desktop app. I do use the Checker Plus for Gmail chrome extension on my main desktop, and now using anything else drives me up the wall because no other extension/application is as fast with notifications, nor are those notifications as useful.
The thing that I REALLY don't understand is using the email address provided by your ISP. Why do people still do this? It's just one more rope around your neck preventing you from switching companies.