Ash's Butterfree was the first Pokémon Ash ever caught. He was released during the mating season when he fell in love with a pink Butterfree.
Butterfree was caught as a Caterpie and evolved to a Metapod in the episode Ash Catches a Pokémon. Metapod then evolved when defending Ash from a flock of Beedrill. He caught it only by throwing out a Poké Ball. He once traded his Butterfree for a Raticate, but he missed his Butterfree so much, so he traded back. It seems to be a male Butterfree and was released to start a family with its mate, a pink Butterfree. Before winning the heart of its mate, Butterfree saved a bunch of wild Butterfree from Team Rocket (this caused the pink Butterfree to start having feelings for him.) Butterfree's current location is unknown. It will return in BW132.
Butterfree is the first Pokémon Ash captured, the first that evolves, the first to reach his final form, the first to be traded away (Ash still got him back), the first to have a confirmed gender, and first to be released.
As a Caterpie, Rikako Aikawa voiced it in both the Japanese and English versions of the anime. When it evolved into a Metapod, though, Aikawa continued voicing it in the Japanese series while Eric Stuart who played Brock in the original 4-Kids series voiced Metapod. Upon evolving into Butterfree, Aikawa began playing the role in both versions once again.