What can be used in place of MSG?

Interesting question -- MSG itself is flavorless; it merely enhances the flavors in other ingredients. As such, the simplest answer to this question is... salt. Salt also has no flavor, per se, and "wakes up" your taste buds so that they experience more of the food's flavor. There are also companies that produce substitutions (see http://www.newstarget.com/021518.html) but nothing particularly popular. Depending on the recipe, salty additives like soy sauce (make sure it's MSG-free, which it often isn't), Worcestershire sauce, etc., can help.

When added to almost to any food, MSG is not flavorless. The Japansese have found a new sense of taste called unami, and MSG will often enhance food by adding this meaty flavor to it. So use meat bases if you want to achieve a similar result.

MSG is Mono-Sodium-Glutamate, usually produced commercially by bacteria. It is converted by saliva to Sodium, (Table Salt) and Glutamate, the anionic form of Glutamic acid. Aside from the flavour enhancement of the Sodium, it is the Glutamate that gives the Unami taste provided by MSG. By itself it has a subtle salty,savoury taste, that is only fully registered when combined with other food.

Unami is a true taste registered by the tongue, not a flavour. As a taste unami is best described as savoury or meaty.

Glutamate and Glutamic acid are naturally occurring amino acids present in a large range of foods. It is free Glutamate that provides the unami taste

They are most concentrated in sharp cheeses (like Parmesan), yeast and yeast extracts, anchovies, fermented fish sauce (and hence Worcestershire sauce), fermented bean products (hence soy sauce), sea-weed/vegtables and savory mushrooms (like Shiitaki). Tomatoes also contain reasonable amounts, which in the quantities of a tomato sauce, have a strong unami taste.

All meat products will have some unami taste because of their high protein content.

As an all-purpose substitute, fermented fish sauce is the best substitute. In the small quantities needed as a flavour enhancer it contributes little extra flavour to a dish other than the desired unami taste. It also contributes salt to the dish. Worcestershire sauce does much the same with a more complex flavour.

Soy sauce is best for Asian dishes.

Parmesan cheese for pastas and other Italian/Western cooking.

Tomato and Shiitaki mushrooms can play roles although they are not as ubiquitous in use as MSG or its concentrated substitutes.

If you want to avoid adding Glutamate altogether, remembering that it is a part of all proteins in every living cell, both plant and animal, you can focus on the other taste and mouthfeel aspects to mask the lack of unami in your dish.

Taste wise you can:

• Add more salt to make you dish saltier.
• Add more sugar, honey or caramel etc... to make it sweeter.
• Add more lemon or fruit juice, tamarind, vinegar or wine to make it more acidic.
• Note: Bitter and alkaline flavours aren't usually desirable but lime juice can give a very nice bitter tang and Bi-Carb Soda is sometimes used in food for its flavour.
  

Or introduce other mouthfeel elements such as:

• Pepper and chilis to add spicy heat
• Alcohol (strong spirits) to add warmth
• Mint (menthol) to add a cooling sensation
  
• Szechwan pepper to add a mouth-numbing sensation