You can download this Y!Go for Nokia 6600 in this address:
There are another links that you can try to download and install to your nokia S60 phone. Please refer to http://en.wikipedia.org/wiki/Symbian.
VERY IMPORTANT NOTE:
Based on Yahoo! Help web site, they already inform us that Yahoo! Go v1.x service was down since August 20, 2007. The consequence with that is all of the Yahoo! Go v1.x application will go down too. Please refer to Yahoo! FAQ.