This answer is mainly to gather a list of interested potential contributors.
Alex Becker: The Complex Real Roots of $x^3-3x+1$. (Level: Moderate-Advanced Undergraduate.) Once you know the quadratic formula, $x^3-3x+1$ is about the simplest nontrivial polynomial you can come up with. However, it is also an example of the casus irreducibilis. All $3$ of its roots are real, and can be expressed in terms of square and cube roots, but not without using imaginary numbers!
Blue: The Seven Faces of a Tetrahedron. (Level: Advanced High School.) A Law of Cosines for dihedral angles in a tetrahedron reveals the figure's three "pseudo-faces", which can be used to devise a variety of formulas, such as a Heron-esque formula for volume in terms of the seven total face areas. (Note: The material lays a conceptual foundation for a topic that extends into Hyperbolic Space at a higher academic level and admits numerous open questions.) [May have to drop out of first wave. Sorry!]
Ron Gordon: Uses and misuses of the residue theorem in evaluating real integrals and sums. (Level: Advanced Undergraduate) The residue theorem is a fantastic tool for evaluating some integrals and sums, and there are many examples in Math.SE of its effective use. Unfortunately, there are also a number of examples in which the theorem, or its applicability, is completely misunderstood. The blog posts will address the use of the residue theorem as a practical tool to evaluate integrals and sums, as opposed to its use because the professor demanded it be used to evaluate such-and-such an integral.
[Ready for posting] Michael Greinecker: Matching theory. (Level: Early Undergraduate) Exposition of the Gale-Shapley algorithm and structural properties of stable matchings. Link
Goos: The coin-minting game. (Level: Advanced High School) Explanation and exploration of a game due to Conway. Symmetries related to the Coin problem; non-constructive proof of a winning strategy; proof that the game always ends despite its highly unbounded nature; open questions.
Daniel Rust: Sturmian Sequence. (Level: Moderate Undergraduate) An introduction to a special class of bi-infinite sequences comprised of two letters and the many ways they can be constructed and ultimately classified. Using continued fractions we can devise a 'test' to see whether two Sturmian sequences are 'equivalent' in a very general sense; links between geometry and combinatorics; open problems concerning higher dimensional analogues. <- I would like to do this post in the future, but I cannot have it ready by the deadline.
anorton: Dial Game (Level: Advanced High School.) Consider an arrangement of $5$ dials, each with $13$ possible settings (labled $0$ through $12$, inclusive). Given a set of possible "moves" (e.g. "move 1 rotates dials $1$ and $3$ clockwise, but $5$ counter-clockwise) and a starting configuration of the dials, what sequence of moves will point all dials towards $0$? Brute force is possible, but would take an exorbitant amount of time. Using linear algebra and modular arithmetic, solving the problem takes relatively little time. <- I would like to do this post in the future, but I cannot have it ready by the deadline.
[Ready for posting] anorton: Area of Polygons (Level: Advanced High School or Early Undergraduate.) Given an ordered list of points describing the vertices of a polygon, one can compute the area. This post will derive such a formula using Green's Theorem. [Side note: I don't really want to do two posts back-to-back, but I would do so if need be.] Link to current version.
Sanath Devalapurkar: Puzzles in the Foundation of Mathematics - Russell's paradox (Level: Advanced High School.) Naive set theory is usually what is (falsely) thought of, by people introduced to set theory, as set theory itself. This post will show one example of a paradox that arises from naive set theory, and lists and provides a basic explanation to a few alternatives to naive set theory, such as $\mathsf{ZFC}$ and topos theory.
[Ready for posting] Mark Dominus (MJD): When do the numbers $A$ and $2A$ have exactly the same (base-10) digits? (Level: advanced high school upwards) I will show how to prove that there are no such $A$ with fewer than 6 digits, and how to find examples with 6 or more digits without resorting to a brute-force computer search. The method I show will explain why all examples of 9 or fewer digits share a certain curious property. NOTICE My article needs editing. Email me if you are willing to read it over.
[ready] vzn. informally/briefly highlight/"gloss over" some advanced/challenging/research math already profiled in personal blog but from an undergrad pov, something like a brief TOC/overview/grab bag of some neat/deep subjects worthy of further study, some tend to cross with CS, some recent breakthroughs in field. namely: Collatz conjecture, Zhang twin prime proof, automated thm proving, Erdos discrepancy problem/Polymath, Erdos 100, P vs NP problem (Claymath prize etc), maybe others.
[First Draft] Jyrki Lahtonen. Two points determine a line, three a quadratic - what has that got to do with CDs? (Level: Advanced High School - Intermediate Undergraduate) An introduction to the algebra of error detection/correction on CDs driven by toy examples. Expected to be ready by the end of May.
[Ready for posting] Paramanand Singh. Playing with Partitions: Euler's Pentagonal Theorem (Level: Advanced High School - Early Undergraduate) Although this proof of Pentagonal theorem by Franklin is well known, I believe I already have my handwritten notes which are elaborate/simple enough to make sense to a high school student. I should be able to put it in blog format by next weekend (17th May 2014). See "Text version on dropbox" and "on stackedit.io"
[Ready for posting] Will Jagy As requested by Jyrki here Binary quadratic forms over Z and class numbers of quadratic fields. About ten pages in Latex, 12 point. No idea what happens next, don't know blogs or mathjax. Also never had any luck with dropbox or similar. Have files BLOG.pdf and BLOG.tex for anyone who does know what happens next and is willing to read the thing. My gmail address is most suitable. Today is 14th July, 2014. Would I lie? Tuesday, 15 July: no further changes came to mind, so I placed the final version, both pdf and .tex, at MMMEEEEEE just under my picture. Meanwhile, mixedmath seems to be at CONFERENCE. Alright, was able to post a draft and edit to some degree. Remaining big problem: I put lengthy computer outputs, I need them to to format the way my C++ program printed them. There does not seem to be a satisfactory blog edit command for a block of multi-line code, although there is a "code" button that does something or other.