The idea of time travel is something that's been used in fiction ever since humans first got a good understanding of time, and anime is no exception to this. I've already brought up two works that heavily involve the subject (Steins;Gate and The Girl Who Leapt Through Time), but there's a few others I can definitely recommend; only problem is, because of the nature of the subject, spoilers are more or less inevitable. Because of that, I'll only be listing series where the involvement of time travel either isn't too big of a spoiler or isn't a spoiler at all, and I won't explain when, where, or how time travel is used, as well as whether I'm talking about traveling backwards, forwards, or just observing different points. Even still, read ahead at your own discretion.
The Melencholy of Haruhi Suzumiya
Karakuri Doji Ultimo
Natsu no Arashi!