Where to publish short notes? Where to publish short notes which don't deserve real publication (an article in a peer reviewed journal)?
I'd press it in blogs but even in blogs supporting math formulas (wordpress.com) it is not very convenient to edit texts with formulas.
As an option: Put a PDF file online and post just a reference to it in a blog.
Better options?
 A: If it is interesting for general audience, Amer. Math. Monthly would be a choice;
If it is also elementary, you may consider Elemente der Mathematik;
If you are patient enough, you may try Mathematical Notes (Russia). 
A: I'm not sure if this is exactly in the frame of your question, but Mathematica could be useful.
You could edit the notes in a Mathematica notebook (a very easy task), and then save them to HTML. By default the mathematics are saved as .gif images, but it does have the option to export to X/HTML+MathML. I think gifs are fine, though.
Here's a screen shot I took. On the left is a Mathematica notebook that I just wrote up. On the right is the output HTML version in a web browser.

As you can see, the reproduction is very faithful.
Here's a picture of Mathematica's "writing assistant" to give you an idea of the sorts of formatting you can do:

And of course, you can always just highlight a piece of a Mathematica notebook, right click, and Copy As LaTeX. Example using the Ramanujan identity:
$\int _0^{\infty }\frac{1+x^2/(b+1)^2}{1+x^2/(a)^2}\times \frac{1+x^2/(b+2)^2}{1+x^2/(a+1)^2}\times \cdots dx=\frac{\sqrt{\pi }}{2}\times \frac{\Gamma \left(a+\frac{1}{2}\right)\Gamma (b+1)\Gamma \left(b-a+\frac{1}{2}\right)}{\Gamma (a)\Gamma \left(b+\frac{1}{2}\right)\Gamma (b-a+1)}$
A: Connexions http://cnx.org/ seems to be something that fits your description. It allows you to upload pdfs etc and I think is exactly for the purpose of sharing such (educational) material.
The mathematica approach seems nice if you want to build a personal website but I'd advise you to put them in a pdf/ps and upload them to some document uploading site even if it is something like google docs. I mentioned Connexions above and there are plenty of such sites around. Just make sure you tag it well and provide a good description so that people can find it.
Also if you are writting in Latex, there are latex to HTML converters 
