Don’t recall where I heard about this but my new favorite universal IM client (i.e. able to connect to most, if not all, IM services like Yahoo!, MSN, ICQ, AIM, etc.) is Miranda. It’s another open source effort with forums for support. Its forte is plug-ins. More than just skins, you can download plug-ins to provide what most other clients include in their basic functionality (like user status tooltips). The basic download comes with support for AIM, ICQ, IRC, Jabber and MSN; if you want support for Yahoo (you guessed it!) you download a plug-in.
Documentation’s sparse but it’s a good, free client and it’s under active development. To get started, download:
* the client — currently version 0.3.1 at about 600K
* the Yahoo! protocol if you’re a Yahoo! user.
* Smiley-Add to add support for familiar, protocol-specific smileys.
* MultiWindow — trust me, you’ll want this.
* Oli’s Yahoo Smileys for a good set of smileys that look like the original ones