Pokemon Yellow offers a few gift Pokemon throughout the game. Some are given after a task is fulfilled, some simply by asking, and some you have to make a choice for. Generation I didn't have the breeding feature introduced in future titles, so this is the only way you can nab any of these Pokemon.

#025 Pikachu

Location: Pallet Town

Method:
The only Pikachu you'll find in the game, this Electric-type cannot be evolved in Yellow and follows you around, similar to Ash's Pikachu on the television show. Unique to this Pikachu is the fact that it has emotions, which can be checked by talking to it. This Pikachu gets Thunderbolt at level 25 as well. To see Pikachu's emotions and what affects them, click here.

Pikachu
Level 5
OT:Yours
ID:Yours
Thundershock
Growl
#001 Bulbasaur

Location: Cerulean City

Method:
A girl in Cerulean City will offer you her Bulbasaur if your Pikachu is in good spirits. The easiest way to do this is to give Pikachu a Potion when at full health over and over again. Pikachu's happiness will climb, and you'll hold onto the Potion. Venusaur, to whom Bulbasaur evolves into, is a fantastic Grass/Poison-type with good Special and balanced other stats that will fair better than a Vileplume or Victreebel on your team.

Bulbasaur
Level 10
OT:Yours
ID:Yours
Tackle
Growl
Leech Seed
#004 Charmander

Location: Route 24

Method:
Right after the Nugget Bridge, a trainer awaits you on a plateau to give you his Charmander. The young Fire-type, who comes very early in the game, becomes a powerhouse after evolving into Charizard. The Fire/Flying-type dragon Pokemon learns powerful attacks like Flamethrower and Slash and has magnificent Speed and Special. Unlike in Red and Blue, Charizard gets access to Fly. A good alternative to Arcanine or Flareon if they don't tickle your fancy.

Charmander
Level 10
OT:Yours
ID:Yours
Scratch
Leer
Ember
#007 Squirtle

Location: Vermillion City

Method:
Officer Jenny, found in the heart of Vermillion City, will hand you a Squirtle after you get the Thunderbadge. Upon evolving into Blastoise, this Pokemon carries very balanced stats with an emphasis on Defense. Blastoise makes for a good alternative to Lapras or Vaporeon should you not be interested in either of those.

Squirtle
Level 10
OT:Yours
ID:Yours
Tackle
Tail Whip
Bubble
#106 Hitmonlee & #107 Hitmonchan

Location: Saffron City

Method:
Received at the Fighting Dojo gym in Saffron City after the Karate King is defeated. Both are Fighting-types and work well on any team, especially against Lorelei of the Elite Four's Ice-types and Bruno of the Elite Four's two Onix. Hitmonchan learns the three elemental punches as well as a few other fist-based moves, though the elemental punches are useless on a Pokemon with such a terrible Special stat. Hitmonlee is kicking oriented and will learn high risk attacks like Jump Kick that have a terribly high base power.

Hitmonlee
Level 30
OT:Yours
ID:Yours
Double Kick
Meditate
Hitmonchan
Level 30
OT:Yours
ID:Yours
Comet Punch
Agility
#131 Lapras

Location: Saffron City - Silph Co

Method:
Given at Saffron City in the Siph Co. room with your rival, Lapras is an extremely capable Pokemon. Water/Ice-type, it learns many powerful attacks like Blizzard, Surf, Thunderbolt, and Psychic. It is recommended that you use Lapras if you want a sturdy Water Pokemon with a good moveset.

Lapras
Level 15
OT:Yours
ID:Yours
Water Gun
Growl
#133 Eevee

Location: Celadon City

Method:
The backdoor of the Celadon Mansion leads to a secret entrance with several flights of stairs. At the very type there's a room where an Eevee awaits, an extremely flexible Normal-type Pokemon. What makes Eevee so special is that it can evolve into three different Pokemon: Flareon, the Fire-type, Vaporeon, the Water-type, and Jolteon, the Electric-type. The method of evolution is via stones, which can be bought on the 4th floor of the Celadon Department Store. Flareon has great Attack and Special, Vaporeon has excellent HP and Special, and Jolteon has superb Speed and Special. A welcome addition to any team.

Eevee
Level 25
OT:Yours
ID:Yours
Tackle
Sand Attack
Growl
#138 Omanyte & #140 Kabuto

Location: Mt. Moon

Method:
The choice between the Helix and Dome fossils is given at Mt. Moon after being the top Super Nerd near the exit. When handed over at the Pokemon Lab on Cinnabar Island, a revived prehistoric Pokemon will be later available. Omanyte derives from the Helix Fossil and Kabuto from the Dome Fossil. Both are decent in their own right, Omastar having superior Defense and Kabutops superior Attack once they evolve.

Omanyte
Level 30
OT:Yours
ID:Yours
Water Gun
Withdraw
Kabuto
Level 30
OT:Yours
ID:Yours
Scratch
Harden
#142 Aerodactyl

Location: Pewter City

Method:
Aerodactyl must be resurrected from the Old Amber, which is found in the Pewter City museum (back entrance, requires Cut). Like with other fossil Pokemon, the item must be taken to Cinnabar Island Pokemon Lab and handed to a scientist before Aerodactyl is given. Aerodactyl is a formidable Rock/Flying-type with high Attack and Speed that can tear through enemy teams with attacks like Fire Blast, Sky Attack, and Hyper Beam.

Aerodactyl
Level 30
OT:Yours
ID:Yours
Wing Attack
Agility