Gmail

Advantages: huge storage capacity, very fast, in-mail chat, searchable Disadvantages: none
When Gmail first came out, people were scrambling left and right to get invites to open their own accounts. The word was out - Google had come out with the best free web-based mail. There were all sorts of hype as to what Gmail had to offer. Very large storage space, filters, labels, searching, no need to delete old emails - all these and more. Now that Gmail has been around for some time, it is safe to say that Google lived up to its promises. Countless people who used to have email accounts with other free web-based email services have had a change of heart and switched to Gmail. Even though you still need an invite to open an account, there are so many invites available that it is not an issue anymore.
Next to the much-hyped up storage capacity, usability is what makes Gmail the best. My favorite new feature is not even about email! Recently, they introduced the Gmail Chat. It is a feature which allows you to chat live with your contacts within the Gmail window. Gmail contacts from your address book are automatically added to your chat contact list on the left hand column. Much like any messenger service, you can see whether they are online or offline. You can even put your own status message. One thing that seems missing is the "invisible" option.
Another thing is that when you are logged out, you cannot receive offline messages. Of course, Gmail is about email really, not chatting.
Speaking of email, one of Gmail's main selling points is that you never have to delete old emails again. Just tag them, label them, and then archive them. Using Google's searching capabilities, you can find any email that you have in your archives. Now isn't that neat?
When you exchange a series of emails with another person, you will get each mail under the same thread. This is much like the threads in forums. The result? You avoid cluttering up your inbox with emails with subjects such as Re:Re:Re:! However, this can be annoying to people who do not use Gmail as they will get a long thread each time.
I also like how much easier it is to view and download attachments. You can even choose to download all attachments in one go. They will be saved in one zip file.
With all these features, it's no wonder many other email providers are following Google's example.