Replace inside Held expression
The Trott-Strzebonski in-place evaluation trick is described here and RuleCondition (internally used but sometimes useful symbol) is mentioned. I referred back to this more than once and I will certainly need to refer to it in the future.
This is a non-obvious but important advanced Mathematica programming technique.
I'd like to propose these two questions:
Data Table Manipulation in Mathematica
Data Table Manipulation in Mathematica: Step 2
where Leonid implements SQL-like 'select' and 'where' functions to handle queries on tables, a very common task.
A random question, namely the topmost on the front page at the time of taking the picture, was selected as the model for this screenshot.
And there y'go! In review of the current volume and the frequency, as well as thoughts expressed now and then from the community, we have decided to instate this direction of a migrational path. Thanks to Shog9 for ...
How does Return work?
Return in Mathematica is different from other languages, and has a quite complex behaviour. Questions about it will keep popping up in the future, so it is worth having and maintaining a canonical answer about it that can be linked to. The description by Leonid is rather detailed and should be included in any such canonical answers....
Why doesn't this use of Begin work?
An explanation of how changing the context affects parsing only. It's a trap intermediate users will run into. I have referred back to this several times, so I would like to have it here locally.
Integrating notebooks to Mathematica's documentation center
This is a rather detailed tutorial on how to create documentation that integrates into the doc centre. It describes a method that doesn't require the Workbench.
It is not a great fit for StackOverflow because it's not about programming.
How to use FORTRAN subroutines or functions in Mathematica?
This is the kind of thing that should not be too difficult for someone who knows C and knows about calling conventions, but it's always good to have a tutorial and save some time.
This question will doubtless have duplicates here on Mathematica.SE eventually, and it is also a question that is ...
Does passing a variable with a large amount of data cost a lot of memory and time in Mathematica?
We got a duplicate of this question here on M.SE just now. It would be easier and more convenient to migrate and update this one than to re-write the whole thing. Otherwise I might just end up copying and pasting the answer, with minor edits ...
As Verbeia has already mentioned, to migrate something to SO, just flag for moderator attention. Regarding the second part of your question — "Shouldn't stackoverflow be an option here? Can we make it so?", adding a migration option is done only if there's sufficient volume to warrant it (even if a site seems like a perfect migration target).
You can see ...
Thinning is a morphological operation on images. It has certain well defined mathematical steps to it, which you can find in the wikipedia article (of course, reading up on more relevant literature/implementation details is left to you). It is by no means specific to Mathematica and an implementation can be found for most languages that have image processing ...