Dusknoir is one of my personal favorites. Any one who has had to battle against mine, named BongoBongo after the Zelda boss, knows this ghost is really tough to beat.
Dusknoir is the Gripper pokémon and is known for it tendancy to take in lost souls into the mouth on it's stomach and carrying them into the afterlife. The antenna on it's head is said to also read transmissions from another dimension. It is a pure Ghost type with great defenses and attack power, an awesome ability, and can play several different roles on a team. My Dusknoir, Bongobongo, is a pretty standard Dusknoir so I will start with that strategy first. I usually lead with Will O Wisp to burn my opponent and lower their attack at the same time. After I get hit I then follow with Pain Split to both heal myself and do damage. I do try to swap the order that I do these moves depending on my opponent, however, as pokémon like Blissey have so much HP that Pain Split makes for a great opener essentially dropping Blissey's huge HP to the same level as Dusknoir. I also use Protect to outlast the opponent and with Pressure, which ups the pp cost of moves from 1 to 2, Protect is a very useful in draining the opponent pokémon of super powerful moves like Fire Blast or Frenzy Plant which generally only have 5 pp (or 8 with PP ups). Sucker Punch is the final move in Dusknoir's list, mostly for picking off opponents once they are low on HP and it goes first if they are attacking Dusknoir. Shadow Punch could be used for this role, and it doesn't require Dusknoir to be under fire to work... but since it can't hit Normal types at all, I tend toward Sucker Punch.
As for other options, Dusknoir's slow speed makes it a great Trick Roomer and it has a wide range of physical moves to use along with Trick Room. After opening with Trick Room, Dusknoir can follow with Fire, Thunder, Ice, or Shadow Punch, Earthquake, etc. depending on what you prefer as Dusknoir has tons of physical attack options.
Another option is to teach Dusknoir the move Imprison (breed with a male Ralts, Kirlia, or Gardevoir that knows Imprison) and three other common moves... for example Earthquake, Sucker Punch, and Protect. If your opponent knows any of the moves in Dusknoir's move list, they are blocked from play. The three moves you choose to block are up to the trainer, I generally try to choose moves that are common among the people I play against.
To get a Dusknoir of your own, you will have to attach a Reaper Cloth to a Dusclops and trade it to a friend and then have them trade it to back to you. There are several places to get a Dusclops, but the easiest way to get a Dusclops is to search the grassy areas at the foot of Turnback Cave in the area known as Sendoff Spring in Platinum. In Diamond and Pearl you will have to use your pokéradar on Route 224 but it's rare so you will have to watch for yellow grass. For both D/P/Pt make sure you are looking at night for best results.
In HeartGold and SoulSilver you can find Dusclops as well, but you need to place 18 Forest Blocks in the Mountain Area of the Safari Zone. If you don't have access to Safari Zone blocks, you will have to talk to Baoba (the Safari Zone Warden) and complete his challenges.
All in all, Dusknoir is a great ghost pokémon and a boon to any team.