Even though the use of sprites for displaying graphics dates back to the late seventies, they are still a recent art form and thus remain contemporary and full of potential for aesthetic and technical innovation. Being so, I prepared a careful selection of online sprite databases for your reference. It is a great journey from the past to our days, regardless of your being a developer or merely an enthusiast. On the other hand, if you are a developer, those are the best of such resources I know and use as reference for drawing my own.
CAUTION: Be aware that the absolute majority of the sprite sheets you will find belongs to others! Being so, be respectful as you use that material as reference. Moreover, you do not want people to look at your work and recognize its been ripped right out of Mega Man, do you? Nonetheless, be creative and good luck!
Bogleech.com's Videogame Bestiary:
The Leaf Forest Zone: