You've probably recently seen anime clips of Hatsune Miku from an anime called Janshin Chan Dropkick, Black Rock Shooter not withstanding, this isn't the only time Hatsune Miku was an actual character in an anime.

For reference, the anime in question Miku is in is known as Shinkansen Henkei Robo Shinkalion.
The anime is about bullet trains that transform into giant robots operated by kids to defeat monsters, the franchise is in direct sponsorship from a toy train maker, so it's an obvious toy anime.
Hatsune Miku is portrayed as a much younger girl of 11 years old with her own Shinkalion train robot based on the H5 Hayabusa from Hokkaido, but her personality is far from the cute care free idol singer we all know her for but rather a cold, stoic, no nonsense character who doesn't really have time to entertain the wide eyed lead characters when they geek out, Miku is also lacking her electronic vocaloid accent as we see her regular voice actress speak normally.
Shinkalion is very much a Japan only anime so many people don't know this bar a few curious people who may have caught it while looking up the Janshin Chan Dropkick scenes where Miku acts closer to her regular personality and even comes with her own Crypton Future Media Manager.
This might not be the last we see of Miku as she may become a character in another project. A couple more anime to drop where Miku has appeared is Sayonara Zetsubou Sensei as a voiced cameo and Aware Meisaku-kun where Miku has her own episode to celebrate her singing the new ending theme at the time.