The Xbox / Xbox 360 has absolutely changed the nature of online gaming. They've done more to make it accessible than ever before: The ease and transparency of playing games with your friends (voice chat, the party system), the introduction of matchmaking in Halo 2 (a huge leap from the 'choose a server' system in the past), achievements, etc, all presented with a unified interface across games.

Both the PS3 and Steam have adopted their innovations - they've become the defacto standard for online gaming.