Yaaga is the online identity I've created for myself by complete chance. I was actually playing some game on my PS2 one day, performing like crap and I needed some sort of war-cry to get myself pumped up to try more - queue "YAAGA YAAGA YAAGA!" (Yeah, I have issues.)
This was the primary use of Yaaga for many months, until it dawned on me when I was registering on a forum that Yaaga sort of kind of sounds like a name, almost. Ever since then I've used Yaaga as an online identity, and the odds are good if you see a Yaaga somewhere it's me. At the least, I've never registered somewhere Yaaga was taken, so it's not that common.
Fun facts about Yaaga - It has practical real world uses, as found out when I tried to google myself one day. It's the name of some tech support company or something, as well as part of some Indian ritual.
It's very easy to mess up apparently. I can't count the number of people that referred to me as Yagga at first...
As for my avatar, it's just a .gif of the scene from early in K-ON where Yui's offering the last available slice of cake to Azu-Nyan, which causes her face to light up. Then she pulls it back and she gets sad. Queue repeating .gif of it. I didn't make it myself, it was found on a thread for pictures of Azu-Nyan in her Fan Club on MAL, someone asked for avatars and this was part of the given list.