Terran and Zerg Edit
When using both the terran and the zerg in a allied or team game, remember that almost all special abilities can affect either player's units. This includes nydus canals and Restoration. Since all zerg units are biological, medics are an excellent way of offering your invasion force an extended lifetime. A single ultralisk can survive almost indefinitely under the protection of a small group of medics.
When well utilized a small force of the combined races has the ability to severely cripple a protoss outpost, all that is needed are a few weak infantry followed by a defiler and a science vessel. After using the defiler's plague ability protoss units and buildings will slowly have their hp reduced to one. Once the enemy reaches low health, use the EMP abilty of your science vessel to instantly deplete all of the protoss shields and psionic energy, leaving them an easy target.
Terran and Protoss Edit
One problem many people find with the terran siege tank is that although it is exceptionally powerful, it is also too slow and takes too much time to switch into siege mode. This dissuades them from using the siege tank in large assaults, leaving the tanks to protect home base. However, if you are lucky enough to be partnered with a protoss ally who owns any arbiters you can bypass that issue. Leave a group of tanks in siege mode and they can be instantly transported to the front lines of the battlefield without the time-consuming changes normally required. Another good reason to utilize this strategy is that the tanks will be cloaked until the Arbiter ceases providing them cover. Unfortunately, this leaves you open to attacks from the air, so it is a good idea to provide cover with Wraiths, Scouts, Corsairs, or Valkyries.