Both Yahoo Messenger and Skype are very common and both have had recent releases of new versions. Also, both offer text chat and voice and video calls. But can Yahoo stand as a serious Skype alternative? Not to my taste, because there are some important areas in which Skype is unreachable.
First, the quality of the calls, in which Skype seems to have some steps ahead. Skype is also well ahead with features, and is also much more advanced in terms of VoIP calling, especially to non-PC communication devices like phones. Finally, Skype is cross-platform, meaning it has support for nearly all common operating systems, mainly Windows, Mac OS and Linux, while Yahoo Messenger seriously lacks support for systems other than Windows. On the other hand, I find Yahoo Messenger better for simple chatters and those just seeking contacts, as it is one of the rare systems that still allow open chat rooms. But then, you know how much filth comes out of these.