GECD is the main resource for career exploration at MIT. Most if not all of the career exploration or recruitment events on campus are posted on CareerBridge, which is under GECD. They also have a grad career news mailing list that you can join, which is really useful if you want to keep informed about the career-related activities on campus (there's always a lot).
MIT Libraries has a page that links to websites about different companies.
The MIT Alumni Association also has career resources, and you can search the alumni database for people who are working in the industry you're interested in. They also run the Externship program over IAP.
IAP in general is a good time to check out career talks or events. There are sessions that give you more information about what's available at MIT, how to use different resources, how to prepare for an interview, etc.
The career fair, especially the one in the fall, is a great place to check out different companies. There are also a lot of career-related events, recruitment and information sessions the weeks leading up to and even after the career fair. Note: MIT career fairs are biased toward tech companies. You may have to look elsewhere for companies that don't usually come here, e.g. non-profit organizations.