As with all proceeding Pokémon games you have the ability to recieve Pokémon via trades with non-playable characters and as gifts as part of storyline. Platinum is no different, you have the opportunity to make four trades and will recieve four gifts as you play through the game. In Diamond/Pearl there are only three gifted Poké,mon while the trades are exactly the same.
The trades, unlike some games in the past, are for Pokémon that you could otherwise capture or evolve, so they may be considered more for the completionist rather than being viewed as necesary. That said these traded Pokémon are a bit rare in the wild or will only be obtainable via evolution until after you get the National Dex. Details on all of these are provided below.
To recieve these Pokémon you will need an open slot in your party. Note also that they will inherit your trainer ID and trainer name, unlike Pokémon that you get through trades.
When visiting Eterna City for the first time Cynthia will greet and, if you have an open spot in your party, give you an egg after you defeat a Team Rocket Grunt. Inside is a Togepi and it will hatch in 2,550 steps (or 1,530 if you have Flame Body or Magma Armor). If you do not have an open slot fear not, she will wait there until you visit her with five or less Pokémon in your team.
After completing all of the things you want to get done in Hearthome City head south-west, toward Route 209. At the gatehouse entrance, if you talk to the man there with open spot, he will give you an egg. Inside is a Happiny and it will hatch in 10,200 steps (or 5,355 boosted steps). The man will wait until you visit him with an open slot if you do not have one the first time you meet.
Bebe lives in the house to right of the PokéCenter in Hearthome City. She is the administrator of the PC system in Sinnoh and will give you an Eevee at a different level depending on which version you have.
In Platinum: You can obtain Eevee the first time you visit her with an open spot in your team. It will be level 20. Your levels should be pretty close to the level of this Eevee and so it could make a timely addition to your team, especially with all of the evolution options available to Eevee.
In Diamond/Pearl: After getting the National Dex return to visit with Bebe and she will give you a level 5 Eevee assuming you have an open spot for it.
Talk to a man in the house north of the PokéCenter in Veilstone City. With an open slot in your team he will give you a level 25 Porygon that he found by the Galactic Building. Your Pokémon may be a few levels above Porygon but it can be a solid addition to your team and you will have plenty of opportunities to obtain the items Upgrade and Dubious Disc which will evolve it.
Leave the port in Canalave City with only five Pokémon in your team, and visit Iron Island for some training before you challenge the sixth gym leader, Byron. Make your way through the rooms until you reach Riley, who will partner up with you to quell some Team Galactic grunts. After defeating them Riley will give you an egg, assuming you only have five Pokémon in your team. If you do not have an open spot you will have to revisit this island and will find Riley waiting in the same spot. This egg will hatch in 6,375 or, if boosted 3,443, steps into a Riolu.
The Pokémon that you get through trades will have the same level as the Pokémon you are sending away, so if you intend to use these Pokémon, it may be a good idea to level up the Pokémon you are sending before initiating the trade event. These Pokémon will keep the nicknames they have as they keep the trainer ID and trainer name of their original owners.
These trades are the same between Diamond/Pearl & Platinum.
For this trade you will need a Machop which can be found on Route 207.
Hilary lives in the first floor of the multi-story house located to left of PokéMart in Oreburgh City. She will offer up an Abra which, without a Quick Ball handy, can be a bit difficult to catch out in the wild.
For this trade you will need a Buizel which are found in the grass outside of the Valley Windworks, east of Floaroma Town.
Norton lives in the Eterna Condominiums, on the first floor, which are situated to right of the PokéMart there. Send him a Buizel and you'll get Chatot, which are common on Routes 213 and 218 in the morning and afternoon.
Norton will also explain the move Chatter, which has been changed since Diamond/Pearl. If you remember way back before D/P were released it was rumoured that Chatter would allow for the recording of your voice and would, when used, then repeat that sound. That didn't happen in D/P, instead you make noise whilst Chatot used the move and, based on the decibals, could confuse the opponent. In Platinum the rumours were justified. Once Chatot has learned Chatter, and it is in your party, select Chatot from the Pokémon menu. The move Chatter will be displayed as would an HM, press A or B and you can record. The recording will then be played when Chatot uses the move in battle. Just realize that you will have to re-record the sound if you deposit Chatot in the PC and withdraw it again.
For this trade you will need a Medicham. You can catch in Mount Coronet by taking one of the southern entrances and making your way to the second floor where Medicham are fairly common. This is much easier than catching a Meditite which can be found just outside of town at levels around 33 and leveling it up.
Mindy can be found in the house in the north-eastern most corner of Snowpoint City. This trade is essentially a good deal as Haunter will not be found out in the wild until after you get the National Dex. You could also catch it in the Old Chateau, if you have Emerald inserted, however. That said, if you wanted this Pokémon on your team you may have already caught a Gastly and raised it well beyond level 25 by now...
Note that Mindy has attached the item Everstone to prevent Haunter from evolving during the trade.
As you make your way from Stark Mountain to the Survival Area, on Route 226, you will Surf passed a house on an island. If you go inside you will meet the Meister who speaks many different languages. Meister will first upgrade you PokéDex so that it can record foreign language entries of Pokémon coming from a select list of regions.
After the upgrade he asks if you have a Finneon. Bring him one, which you can find by fishing with a Good Rod on Route 218, and he will trade you a Magikarp. This Magikarp will enable the German language entry of Magikarp in your PokéDex.