Windows/Mac/Linux: At the very least, the free, just-released Zimbra Desktop client gives non-paying Yahoo Mail users
IMAP-style offline access to their messages. More than that, though, Zimbra adds a few features to Yahoo, Gmail, AOL, or any other POP/IMAP account that could be pretty useful—at least at some point down the road. Message tagging and nested conversation arrows are pretty nice features for any services that don't already have them, but they don't sync back, or work with Gmail's labels. There's also a bare-bones word processor/spreadsheet, a (non-syncing) calendar, and more search options, including attachment filtering. Overall, though, the real benefit is Zimbra's ability to synchronise your outbox and mail folders before you go offline. Zimbra Desktop is a free download for Windows, Mac OS X, and Linux systems.