This is shi: シ
This is tsu: ツ
I see lots of people using these as smilies, and I can see why. They aren't meant to be, though.
They are Japanese katakana. Katakana (カタカナ) is one of the three systems of writing in the Japanese language, with the other two being Hiragana (ひらがな) and kanji (漢字).
Just putting that out there!