The key to defending against this strategy is fairly simple. For Terrans, place Bunkers and a few Missile Turrets around the resources and town. Protoss players should place Photon Cannons around the resources, or use groups of Dragoons. Later on they can use air units such as Carriers and Scouts for defense. Spore Colonies are the best anti-drop defense for Zerg. They provide cloak detection, have a pretty long range, and do considerable damage to Dropships (or other transports). If the enemy is able to land Dropships without being attacked, they will easily be able to destroy a town fairly quickly. Expansion towns are especially vulnerable to this type of attack. This strategy is also something to watch out from the Protoss and Zerg Races. Protoss players can unload 4 Zealots per Dropship or 2 Reavers while Zerg players can place 8 Zerglings in an Overlord or use 4 Hydralisks.

Million Man Marine March
This strategy involves producing a large amount of un-upgraded Marines. Build a Supply Depot at about 8 to 9/10 supply. Continue making more SCVs, and build a Barracks. Build another Depot, and a second Barracks. Continue making more SCVs, Supply Depots, and build another two Barracks to bring the total to four. Start building Marines, but be sure to evenly distribute the Marines across all four Barracks. For example, build 1 in the first one, 1 in the second, and so on. The idea is to have all four Barracks building constantly. To make this easier, assign the Barracks to control groups 1-4. Make sure you Rally all four Barracks to the same location, usually near your SCVs mining Minerals to protect them. Once you've assembled a large group of Marines, you can charge the enemy. This should work very well vs Zergling rushers, Hydralisks, and Zealots. Build a Refinery, Academy, and Engineering Bay to research Stim Pack, U-238 Shells for added range, and upgrade your Marines to add to their advantage. Stim Packs are the most essential upgrade, as it doubles the effectiveness of your Marines. Add in Medics as soon as possible for an even better combo.

Counters:

Zerg
Your best bet is a lot of Hydralisks from at least two Hatcheries. Make sure you research Grooved Spines in the Hydralisk Den to give your Hydralisks a greater range. If the enemy insists on continuing to build nothing but Marines, build Queens and cast Ensnare on the Marines. This should allow you to overpower them without taking severe losses. Then build Guardians which should be able to fire on the Marines at greater range, killing them easily and quickly in numbers.

Terran
A nice thick defense of Bunkers filled with Marines should allow you to defend your town until you can make Siege Tanks or build lots of Vultures, which are very effective in countering large amounts of Marines. Research the Spider Mine upgrade and start laying Mines also. Be sure to repair your Bunkers, Tanks, and Vultures to keep them in full form.

Protoss
Build 3-4 Gateways and a lot of Zealots. You can use multiple Shield Batteries to extend the life of your Zealots. If the enemy continues with this strategy, quickly build Reavers with the Increased Reaver Capacity upgrade. You can kill as many as 8 Marines with one Scarab if you are lucky, and a pair of Reavers can wipe out huge groups of Marines. Archons also work very well against Marines. While you are building Zealots, you often save up a lot of gas. Use this gas on Templars. Cast Psionic Storm on groups of Marines, then use Templars with no mana remaining to make Archons which will quickly work through the large groups of Marines.