This site allows you to find best rates for roaming within Europe. Simply select the country of your origin and the country you're planning to visit. http://www.roaming.gsmeurope.org/
Good site for Europeans! But I live in US and I need to travel to Europe. What I know for sure is that roaming services are very expensive for all users around the world, regardless of the mobile provider they use. My choice fell upon CallinEurope.com, they achieve the same effect as international roaming with a major difference – they are much cheaper.
The site seems very promising for US user to provide cell phone services while traveling Europe. i have had paid outrageous roaming charges on my way to my country through Europe. My few minutes of Hi and bye cost me equivalent of one month's of cell phone upkeep.