You may have heard many different things about a NIT during your time in El Salvador. A NIT is similar (in some ways) to a Social Security Card in the USA.
That is to say, the NIT is used for tax purposes. You need a NIT to make large purchases in your name, run a business, import items (requires NIT de importador), etc. Many people find the need for a NIT when importing items from customs, purchasing a car or property in El Salvador.
Anyone can get a NIT. You do not need residency or citizenship in El Salvador. To get a NIT all you need is your US Passport (or Passport/Passport Visa) from your country. You will take your passport and visit the office to get a NIT. The offices are located at "Las tres torres" in San Salvador. This is the office I recommend. There is also an office in HiperMall however they have weird hours and seem to have slow service (although people swear it is faster).
The office behind Las Tres Torres is a medium sized office. Parking is available outside on the street. They charge $1.00 to watch your car. It is paid upon leaving. You will enter the office alone. If you do not speak Spanish well, you will need to wing it. They are very particular about letting someone in with you -- even a translator due to limited space. Once you go in there is a reception desk. The lady will ask if this is your first time for a NIT (primera vez) or if you are renewing. You will tell her first time, she will point you to the back of the building (in sight) on the right side. There is a line and a window. You will be asked if this is your first time or renewal. You will tell the lady first time (primera vez) and then pay, it is about $2-3 USD at this time. You will then return to the front and to the left of the reception desk is an area that says copies (Fotocopias).
You walk to the photocopy desk and pay $0.20 cents and hand them your passport. They will make a copy of your passport. Directly in front of the desk you will then notice a line, you will join this line and wait. Once you get to the front you will be called to one of many computer desks and you will hand the gentlemen your passport copy and colored paper slip. He will fill out a sheet, ask you to sign and then you will stand up and return to the right of where the photocopy desk is located. You will see other individuals standing there. This is where the NIT's are dispatched.
You will be called by first name. Once your name is called you will walk to the desk. You will be handed your actual NIT which you will sign the back of. You will then turn around and return to the photocopy desk (right behind you) and pay about $0.60 cents to have the NIT laminated. That is all. You're done.
Enjoy your new NIT. The total cost is about $4-$5 USD. I recommend you bring change to make paying for the copy and lamination easier or bring one dollar bills.
The entire process takes 20 minutes on a crowded day, much less if it is not crowded. Avoid going from 12-1 as you may catch them on lunch. They open between 8-9 AM and stay open until 5 PM but don't expect much after 4PM.