Google is playing for keeps in the email service arena these days and to prove it they just added a free Gmail IMAP functionality:

Gmail has allowed access via the web interface or POP access for quite some time now. POP allows e-mail clients to download messages from the server, but doesn’t reflect any changes on the server once the messages are manipulated on the client side. So if you download five messages, read four of them, and move three of them to other folders on your desktop e-mail client, those messages will remain unread and unmoved on the Gmail server. When you check the server again from a different device, you have to go through the whole process all over again with the same messages.

Such is not the case with IMAP—any changes you make on the client side are synced back with the server (when a connection is available), so that read items remain read and moved items remain moved on all devices checking that account. In other words, IMAP treats remote folders as if they were local, which is great if you use more than one interface for accessing and organizing your email (say, webmail from work, your iPhone on the road, and a mail client like Thunderbird at home).
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Gmail Product Manager Keith Coleman has another theory on why webmail services haven’t made IMAP widely available, noting that most (including Google) are at least somewhat dependent upon advertising revenue from their web-based clients. “We thought that was a trend worth breaking,” he told Ars. “Initial reaction has been great so far.”

It’ll be interesting to see if this results in a substantial bump in the number of Gmail users. The number of folks who need or want IMAP is probably small, but still significant and they’re currently the only web based email to offer both POP and IMAP connectivity.