I have tried Google Talk (GT). It seems to be a good competitor of SkyPE. We can see more features available at SkyPE but in fact, I love GT more than SkyPE. It has a tidy interface which most of other IMs are lacking of..

GT is based on an open technology called Jabber which was created to help different networks talk to each other. Georges Harik, director of new products at Google, hinted that this was its aim when the Talk service was unveiled. "We are going to try to be the first in the world to connect everyone to everyone," he said.
However, Google Talk does not yet use these features of Jabber and, for the moment, is as closed as any other network. Similarly, its net telephone service currently works only for GMail account holders.
On net telephony services, it also has a long way to go to catch up with rivals not least because managing a system that can provide good quality phone calls is very different to letting people scour a database with search queries.
If you want some experience on Google Talk, drop me a note on Yahoo!, MSN, SkyPE, AOL, ICQ or IRC. I will send you the Gmail invitation and we'll talk about GT.
Google Talk is available at:
Google Website - http://talk.google.com