Gmail was the first online mail service built as a service, rather than a website. It revolutionized the field of e-mail - not only did it offer a relatively dizzying amount of storage for those days (about a hundred times larger than any other service that existed up to that time) Gmail is the brainchild of a Google engineer named Paul Buchheit, and started as a failed side development project of his that started back in 1996.
After a month or two of working on the new project Caribou, Gmail was launched in August 2003.