Charizard: he dropped it off at the Charicific Valley to become stronger (I don't consider it a loss, because he can get it back just by giving Liza a call)
Butterfree: He sent it off for breeding purposes
Primeape: He sent it off to train with a real fighting-type trainer (I really didn't get it :\)
Beedrill: He gave it to Casey because she likes yellow and black striped pokemon...
Squirtle: He sent it back to the Squirtle Squad to help them win a pok?mon firefighting competition...
Lapras: he returned it to its family
Pidgeot: He left it behind to watch over the pidgey and pidgeotto of the Viridian forest
Larvitar: He had to ruturn it to its mother (but it was never really Ash's pok?mon to begin with)
Pok?mon at Oak's: includes Kingler, Muk, Tauros, Snorlax, Heracross, Bulbasaur, Bayleef, Cyndaquil, Totodile, Noctowl, Phanpy as of the episode "Gotta Cath ya later"