Taishou.a - anNina (Higurashi no Naki Koro ni Kai ED) Vermillion - Ishikawa Chiaki (Bokurano ED 2)
...are the ones that first come to mind. I really like the feel/sound of these songs, and I would love to hear more that are like them.
Some I'll include because they're so catchy:
Rinbu Revolution - Ouki Masami (Revolutionary Girl Utena OP) Pegasus Fantasy - Make-Up (Saint Seiya OP 1) The Omega version is good too. With such simple lyrics, it's easy to get caught in your head. Hyadain no Kakakata☆Kataomoi - C - Hyadain/Maeyamada Kenichi (Nichijou OP 1) Dangerously catchy.
My favourite would have to be Parallel hearts by FictionJunction from the anime Pandora Hearts, or the theme song (which i don't have a clue what it;s called -_-, out of curiosity anybody who knows please tell me, i'll be much obliged d(^^) From Hakuouki. (Probably the episodes >20)3nodding