Command & Conquer 3: Tiberium Wars (PC)

Secret Scrin Campaign
To play the secret Scrin campaign, finish both the GDI and Nod campaigns - though that may be difficult for some gamers as it can be a tough game at times. If you do this, you can unlock the full Scrin campaign that spans four missions. Cool.

Mammoth Tanks
Though the Scrin and NOD have some fancy units capable of dealing out the damage, the Mammoth Tank is arguably the best unit in the game. It's only downfalls are it's slow moving pace and expense. Otherwise, it's awesome! If you are GDI, simply built squads of five or six Mammoth Tanks and they can usually provide a strong defensive/offensive option to have in reserve, attacking both land and air units with dual 150mm cannons and 4.75-inch guided rocket pods. The main cannons can also be upgraded to railguns, which give the tanks immense stopping power.

The best option is to capture some enemy buildings when they build in a new tiberium field with only a few units (always have scouts searching tiberium fields for activity), then sell the buildings and queue up half a dozen Mammoth Tanks. Happy hunting.

Play as NOD (C&C 3 demo)
At the skirmish options menu, hold N and click you army to make NOD as selectable option.
To switch back, hold G and click your army to make GDI selectable again.
This works with the computer army as well.