Nimbuzz is the new Skype for mobile phones – free phonecalls and text messages to other mobile phones connected to the Nimbuzz network (over GSM, not GPRS/UMTS/etc). It seems to be out for a lot of brands, including a version for the Palm Tungsten C as well as the usual crop of Nokias etc. It also comes for Windows PCs. Unfortunately I couldn’t get it for my Treo, so can’t tell you much more about it, but this looks worth following, as Skype isn’t looking at Palm at all yet.
It’s all still in alpha / beta and you download it using your phone’s browser.