When someone mentions tsundere, I think of Queen Kugimiya Rie's top 3:
Shakugan no Shana
Hayate no Gotoku
Zero no Tsukaima
Kugimiya is the queen of tsunderes, so you should go look up all the series she did the main characters for (except FMA haha, Al isn't exactly tsundere)
Then Lucky Star also comes to mind (that one's a masterpiece, serious) XD
Just to help a little, I'll put a list of the ones I've already seen with a tsundere in it. I'll have a "*" next to ones I liked and a "**" next to ones I loved.
Baka and Test: Summon the Beasts **
Gosick **
Yuru Yuri **
Shakugan no Shana + II **
Ookami-san and Her Seven Companions *
The Familiar of Zero/Zero no Tsukaima
Hayate the Combat Butler/Hayate no Gotoku *
Toradora! *
Angel Beats! ** (You could say Yurippe's a tsundere.)
Clannad + ~After Story~ **
Kanon (2006) **
Pandora Hearts **
Hidan no Aria/Aria the Scarlet Ammo *
InuYasha + Final Act **
Hoshizora e Kakaru Hashi
IS: Infinite Stratos * (Wasn't a fan of the tsunderes in this one, though. Only Charlotte.)
The World God Only Knows + II *
Toaru Kagaku no Railgun *
Toaru Majutsu no Index + II **
Kampfer *
Lucky Star**
Kore wa Zombie Desu ka?/Is This a Zombie?*
Negima! Season 1, 2, OVAs and OADs *
Oreimo *
Ouran High School Host Club *
The Melancholy of Haruhi Suzumiya *
Yokuwakaru Gendaimahou *