Gmail is without a doubt one of the most popular and most well-known services on the Internet. Starting as an invitation-only private service with less than 2GB of storage space and limited integration, it has ballooned into ten times that and more. With its millions of users, tight integration with other services and vast improvements in reliability, the free web-based email service is definitely a shining of example of what Google can do right. And it's been doing that for some time now, with Gmail officially turning five years old as of yesterday.
Despite this, Gmail still carries the "beta" tag it did on day one. Most Google services do, for one reason or another, but nobody seems to pay attention to that anymore. From Gmail's simplicity in basic functionality to its richness in advanced functionality, it has been a very successful tool for them, and has certainly given a lot to people who rely on web-based services. You can easily draw a parallel between Google offering Gmail and other services (Hotmail, Yahoo, etc) quickly expanding their own set of tools, ranging all the way from Inbox storage size to spam filtering. Will Gmail still be around in five years? That will most likely be the case, though I wonder what sort of changes we can expect from the service in that period.