I've played some awesome games over the years online, with great memories in all.
Runescape, Flyff, Guild Wars (and all expansions), WoW, and less time in others.
Currently I'm playing Runescape and I'm psyched for Guild Wars 2.
Xbox Live is always great too, I've been playing Perfect Dark on XBLA with my friend on occasion, and NHL 10 and NHL 11 were fun as well.