The use of っ (tsu)
This has been driving me mad ever since I learned how to read Hiragana.
がんばってくださいね is pronounced "ganbatte kudasai ne", right?
Why is there the っ (tsu) there if you don't pronounce it? What purpose does it serve?

Question

The use of っ (tsu)
This has been driving me mad ever since I learned how to read Hiragana. 
がんばってくださいね is pronounced "ganbatte kudasai ne", right? 
Why is there the っ (tsu) there if you don't pronounce it? What purpose does it serve?

Answer

large『つ』　and small『っ』

『がんばつて』----->ga n ba tsu te
『がんばって』------>ga n ba tte

『あさって(asatte) the day after tomorrow』

がんばってくださいね！

Answer

Just like everybody said, it doubles any sound that goes next. (「っか」- kka, 「っぽ」- ppo, etc.) And if there's no more kana next, it just reduces (or strenghtens?) the tone of previous sound (「えっ」- Huh?) or halt it like "..." in the middle of a word. [But that's just what I think; correct me if I'm mistaken]

Answer

It serves to double the "t" of "te", thus making it "tte". Come on, man, that wasn't hard.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!