2 rax, no gas
This indicates either a fast expansion or a scv/marine all in. Firstly, you should immediately throw up a bunker when you see this. That's just obvious. But what do you do afterwards? If your opponent is halfway decent he won't commit all his scvs/marines knowing that it will fail (for example, if he scouts the bunker). Most players would just expo and play standard from there. You might think that you'd have the advantage but if he doesn't commit his scvs, it's pretty much an equal battle. However, there is a strategy that will completely crumble the 2 rax opener if all goes well. The ultimate composition of...
Marines, Reapers, Scvs!
Note: I created this on the fly after seeing my opponents unit composition. It was the ultimate eureka moment. As such, I don't have a refined build order.
Why this works...
Your opponent is unlikely to have much gas, and he will also have numerous barracks. This limits his production to marines/marauders. The reaper is the key unit to this build as the ultimate DPS bringer. Reapers absolutely devour marines, and not only that but they devour structures as well. This means that your opponents pesky bunker/marine/marauder force will crumble with a few reapers. The only downside is reapers have low hp themselves and it won't be game ending unless you bring your scvs. Alongside your leftover marine force from the early stages of the game, your reapers are the pain givers whilst the scvs tank the damage.
Here is a replay that shows how dominating this really is:
I want to point out that in the replay, I played quite cautiously. I didn't blindly start making reapers. If my opponent had played standard, this strategy would most likely not work. But I knew he didn't have that much gas, meaning no factory or starport. You'll notice as well, at 6:45 I throw down a scan and from that I concluded that he only had barracks units and decide to fully commit with my scvs.
Feedback would be nice, as well as any holes in the strategy. Obviously there a lot of circumstances such as if he fast expos and then suddenly decides to go gas heavy. It really comes down to scouting their production buildings before you fully commit. If feedback is positive, I'll try to refine the build and get a specific BO.
Also, since I know someone is bound to ask... I'm a 2.6k diamond random player on the NA server.