I love tsundere animes but I heard rumours that Rin is tsundere too? I've seen Shirou save her life a couple of times but I never see them have interest in each other like Shana and Yuji in Shakugan no Shana or Saito and Louise in Zero no Tsukaima.
Fate/Stay Night is based on an eroge Visual Novel (type of PC game popular in Japan). In this game, there are 3 different main routes. One for Saber, one for Rin, and one for Sakura. Depending on which route you take, defines what romantic interest you will get.
The anime adaptation, whilst being a bastardization of all three routes, was primarily focused on the Saber route. But even in this route, there are times that Rin does still show affection for Shirou. And yes, Rin is a quintessential tsundere.
However, there is a new Fate/Stay Night movie coming out in January that is based on the Unlimited Blade Works route (the one where Rin is the love interest). This is pretty awesome, because most of us agree that UBW was the best path in the game. http://myanimelist.net/anime/6922/Fate/stay_night:_Unlimited_Blade_Works