Gah... these are in no order, I'm just typing as they come to my head. I'll be surprised if I even come up with 20.
1) Shuichi and Yuki (Gravitation)
2) Kouta and Lucy (Elfen Lied)
3) Kiba and Cheza (Wolf's Rain)
4) Sakura and Dokuro (Bokatsu Tenshi Dokuro-Chan)
5) Ashitaka and San (Princess Monoke)
6) Naota and Haruko (FLCL)
7) Haku and Chihiro (Spirited Away)
8) Jack and Sally (Nightmare Before Christmas)
9) Yuugi and Atem (Yuugiou)
10) High Priest Set and Kisara (Yuugiou)
11) Kyou and Tohru (Fruits Basket)
12) Raven and Malchior (Teen Titans)
13) Raven and Rorek (Teen Titans)14) Raven and Beast Boy (Teen Titans)
15) Raven and Jinx (Teen Titans)
16) Robin and Slade (Teen Titans)
17) Daisuke and Dark and Satoshi (DNAngel)
18) Howl and Sophie (Howl's Moving Castle)
19) Ulrich and Yumi (Code: Lyoko)
20) Remy and Rogue (X-Men: Evolution)