Skype alternative for Android devices: http://www.fring.com
AFAIK there is no PC version for it.
Yeah, there are a few (I've tried Nimbuzz). The problem with all these alternatives to Skype is that they have a tiny user base in comparison to Skype, so they are really only of use if you want to use them to call a land line or mobile. And even then, the number of countries supported is not as extensive as with Skype, as far as I know.
I can only hope that, as Android is far more popular than Windows Phone 7, Microsoft do not cut off their nose to spite their face. Skype for Linux is another kettle of fish entirely, as the number of Linux desktops
is minuscule in comparison to Windows desktops (and the number of Linux desktops using Skype is even smaller), so Microsoft may well let the Skype for Linux application wither and die, as the loss of revenue would be minimal in comparison to Skype for Android. By the way, my main use for Skype is Skype-to-land line and Skype-to-mobile, from one country to another. I do use Skype-to-Skype occasionally, but normally I'm paying for the Skype service so I can call from one country to another one at local rates. That is the benefit of Skype to many people, and Skype's business model isn't built on free Skype-to-Skype connections.