Minecraft can be played online and both genders like it. FarmVille and the like on Facebook. It depends on what you like, there's neopets and such but that's for younger people. I love the browser game Kingdom of Loathing but it's for adults, not 10-15 year olds. Addictiggames has good free games in-browser but have a firewall and antivirus/malware because the ads there are suspicious.