Wikipedia, Wikiversity, Wikibooks.
Basically, this would be a directory of equations and mathematical theories. This could be useful to everyone from school students looking up the formulae required for an exam to maths professors collaborating on a proof. It also helps that verifiability would be relatively simple—a proof is a proof.
Proposed by 
Proposed Features (by manifold) 
- For it to work, a logic programming language similar to mathematics has to be created, with the following features
- it should be very similar to write mathematics formally correct
- rendering to latex code, and to html
- language is understandable by the computer, and so the steps of the proofs are verifiable by the system
- Every users has the ability to enter/edit a mathematical proof, building a mathematical library
- Steps of the proof are linked to basic axioms and other theorems; missing theorems are highlighted red
- The system checks the steps (since they are linked to existing theorems, or marked as "red"/"Missing theorem"), and therefore accepts the proof (optional) an describing text to the theorem
If successful, the database of theorems, could be used to:
- A theorem and proof library
- search theorems related to one another (making different proofs visible by showing the different dependencies)
- create a powerful tool to filter thousands of theorems with key ingredients (like: Hilbertspace and Operator and Dual space)
- conclude further theorems, and proof unsolved problems (like [en.wikipedia.org/wiki/Vampire_theorem_prover Vampire theorem prover]).
- automatically test a Lemma
- If successful, forks for theoretical physics, theoretical chemistry and so on would be created and would provide a huge benefit to these fields
Alternative names 
- Wikimaths, with an S (I actually prefer this one, being British, but I expect the version without the S to be more popular, being more American)
Related projects/proposals 
- Wikispecies is similar in that both projects focus on quite a specific audience—in Wikispecies's case biologists, in this case mathematicians (although I believe the appeal of this project would be somewhat wider than that of Wikispecies)
- There may be some overlap with Wikiversity, but I'm not sure of the extent.
Domain names 
- http://wikimath.org appears to be for sale, although this is through a third party, rather than a registrar
- Since mathematics is universal, it may not need language-specific versions, so http://wikimath.wikimedia.org may be sufficient. This is already done for Commons, Wikispecies, etc.
- I can't find any active wikis in this area. Something called Wikimaths does exist, but it has very little, if any, real content.