Marmite is made from leftover brewers yeast, it is popular in South Africa, Australia, etc.
Marmite is made from yeast extract, a by-product of beer brewing. (Wikipedia)
So would it be considered halal, or is it haram?
Marmite is haram. Eat vegemite instead!
Kiwi-bashing aside, from their FAQ:
Is Marmite certified Halal?
Yes – The Halal Certification Authority of Australia has certified Marmite as a Halal product.
Is Marmite kosher certified?
Yes – Marmite is Kosher certified.
Is Marmite suitable for vegans and vegetarians?
Yes – Marmite is vegan and vegetarian friendly.
This same problem arose in Haraam or Halal? - Coke and Pepsi contain 0.001% Alcohol, where it was noted that e.g. Coke contains minute amounts of alcohol. So I reiterate the main ideas of my answer in this context:
If a substance is incapable of intoxicating even in large quantities, then it is not haram (insofar as to being an intoxicant; it could still be haram for other reasons).
Some people think that the words of the Prophet (peace and blessings of Allaah be upon him), “Whatever intoxicates in large quantities, a little of it is haraam”, mean that if a small percentage of an intoxicant is mixed with a large amount of a substance that is not intoxicating, then it is haraam. This is a misunderstanding of the hadeeth. ... if something is mixed with alcohol but the alcohol is a small amount and does not have any effect, then it is halaal and does not come under the ruling of this hadeeth. -- Islam Q&A, in the context of non-alcoholic beers
The prohibition applies to the substance as a whole, not to it's individualized parts:
Sheikh Salman al-Oadah astutely observed: "The percentage of alcohol mentioned has no effect on the ruling. The ruling applies to the drink itself taken as a whole and not to its composition." -- IslamToday.net, also in the context of non-alcoholic beers
It's reasonable to believe that even a large amount of Marmite is incapable of intoxicating, and thus prohibition due to being an intoxicant does not apply.
Anything that can make you intoxicated is not good. The key point is effect of the substance not what it is made from.