I've seen 3:
Ayu
Hangry & Angry
Gackt (Yellow Fried Chickenz)
All this year as well. 2010 has been the best year for concerts EVER XD
I'd like to see:
BoA
L'Arc~en~Ciel
Utada (almost did but I got outbid on eBay for tickets

)
Morning Musume
Koda Kumi
Namie Amuro