At first I used YPOPS!, a program that read the HTML pages provided by the Yahoo webmail client and served the emails on a local POP server. Cool, right? Except it always broke whenever Yahoo changed their pages around.
After a bit I moved onto using the WebMail extension for Thunderbird. It essentially works the same way, but runs in the Thunderbird process as an extension instead of as a separate service. It's a little complicated to set up and requires two extensions. The general WebMail extension and a Yahoo specific one (there's also ones available for Hotmail, Gmail, Libero and AOL). You might wonder why this exists for some services that have always had POP/IMAP access (Gmail)? I was too, apparently some people cannot check those ports because of firewall issues so this essentially allows them to check it via port 80, over an HTTP connection.
A few months ago I came across post describing how a simple (nonstandard) command could allow access to the Yahoo IMAP server, but it required patching the Thunderbird source -- not an option for me since I've been running nightlies of either Thunderbird 3.1 or 3.2/3.3 for a while now. (I mean sure, I could do it...but WebMail extension was working fine.) There was some discussion about it and a bug was filed for Thunderbird.
So how did this lead to free IMAP support? I noticed in the Weekly Status Meeting Notes (2010-11-16) for Thunderbird a mention of Free Email Providers page. Checking it out it said "Y! Mail is a free email service provided by Yahoo! It offers webmail supported by targeted advertising as well as IMAP access." Hmm...but I just said they don't have support this for free! A quick Bing search brought up a page with IMAP server settings. I figured I'd check if they work, and sure enough they did! The settings are copied below:
User name:Awesome! Anyway, I replied with this info in the aforementioned bug and a patch was quickly added by Mozilla's Ben Bucksch to support this in Thunderbird, hopefully it'll make it into the next version! It was also brought to my attention that imap-ssl.mail.yahoo.com provides an SSL certificate that is valid for imap.mail.yahoo.com only, I'd suggest using that former.
SMTP server: smtp.mail.yahoo.com
Note that I'm currently suffering from another bug while using Yahoo IMAP. Everything works, there's just an annoying pop-up occasionally about the error. Hopefully it will be fixed soon.