Google has at last opened up its well-regarded email service to all comers, eliminating the need to receive an invitation from an existing user before signing up. It's been two years since the service launched, and it's still branded beta. The primarily Web-based Gmail remains free to use and  for each user, an amount that grows every day as Google brings more storage online.
Gmail provides POP3 access for people who prefer using traditional email clients over the Gmail Web interface, but still lacks IMAP, a popular alternative to POP3. The service has good spam filtering that accepts reports from its users, attachment viewing within the Web interface, and support for mobile phone email - you can read and reply to messages. Google Talk, the company's instant messaging and voice chat program, can be used within the Web interface, too.
Gmail can also forward incoming mail to another email address, send messages and replies using another return address, and fetch mail from up to five other non-Gmail accounts via POP.
 from other email services and clients is how rapid-response searches sit at the core of message archiving and organizing. Instead of filing messages in a folder, you apply one or more labels to incoming messages that exist in a general archive. The labels are the search equivalent of folders, and show up in a list in the left navigation bar. It's very much like the Smart Mailboxes feature in Apple Mail, only faster. You can also search quickly through your entire message archive; Gmail provides a list of matches with search terms highlighted, and messages threaded across an entire set of conversations so you can easily follow what was said and by whom.
Gmail funds itself through what text ads that are theoretically relevant to the content of the message you're reading - a behavior that can be a little disconcerting. The ads are never inserted into the body text of either incoming or outgoing mail, however, unlike free mail from other firms.
Since I run my own mail server, I have only dabbled with Gmail. But reports from friends and colleagues who rely entirely on the service are highly positive, with them experiencing minimal downtime and speedy access. The main reason I haven't looked more seriously at Gmail is that I already have 2.3 GB of stored email, and I don't want to lose access to that archive. Gmail does enable import of contacts to provide access to laboriously created address books.
(The only way to import mail into Gmail that I can see is to redirect to the Gmail account, which isn't practical with hundreds of thousands of messages. Although utilities like Mark Lyon's free and Cheah Chu Yeow's free  exist to automate the process, date stamps are lost in the transfer, rendering them useless for an email archive that spans 15 years.)
Because Gmail is an extremely interactive Web application, it works only in a subset of common Web browsers, including (all version numbers are the earliest supported version) Safari 1.2.1, Firefox 0.8, Mozilla 1.4, and Netscape 7.1. Web-based chat requires Firefox 1.0 or later. Gmail does offer a basic HTML view for other browsers, but, honestly, there's not much point in using Gmail if you can't take advantage of the full interactive interface.
If you've been looking for another email service, it's worth giving Gmail a try. You can even set up multiple addresses using your own domain using the service. In particular, if you've been having trouble receiving TidBITS due to overactive spam filtering or other delivery problems, Gmail could be a good alternative. (Our new bounce processing code sends you warnings when your account has bounced too many messages, and you can manage your subscriptions and addresses with our new  interface.)