Cheaper products for consumers and lower cost for companies. 
The negative externalities, and one that is reiterated in all news outlets and by various politicians (specifically in the US), is the loss of domestic jobs. I have an issue with these media outlets and politicians because they don't understand WHY domestic workers are losing their jobs. 
Low skilled labor (jobs) get exported to China, India, Singapore, etc. while high skilled labor stays in the advanced economy. Hence the reason why Sillicon Valley attracts the best computer scientists, programmers, etc. and Wall Street attract financiers. 
Looking at other countries, the same observation can be seen as well; Germany is highly advance when it comes to capital equipment, England is highly advanced when it comes to the financial industry, Switzerland has highly advanced scientists, etc. 
This whole argument against free trade baffles me because it makes it seem that no high wage jobs will be available in the future. I'm not saying that it's not going to be easy for the displaced worker, they have to endure the hardship of losing his/her job and now has to find a new one (sometimes in a completely different field). And I'm not saying it's going to be easy and expedient for the displaced worker to find a new job either. 
What I am saying is that international trade benefits the greater good and makes the creation of goods/services more efficient and cost-effective.