Links and some random things that didn't seem to fit neatly into any other tab on this page.
SIAM (Society for Industrial and Applied Mathematics ) I used to be an active participant in this professional organization, serving as secretary for the DC-Baltimore region when I was a professor at the Naval Academy and starting a student chapter at UW-Madison during my time in graduate school.
I use homebrew for my Mac machines. You can use that to install important software for writing code such as GDB and modern
LaTeX is a typesetting language used to create modular documents with beautfiful equations. There is an excellent community of very helpful people if you run into questions about what they do, and stackexchange is an excellent source of information
Windows. Download and install MikTeX. Then you'll need a text editor to write all your stuff. My favorite used to be TeXnicCenter, but I understand that TeXmaker is much cleaner for Windows platforms.
Macintosh. TeXShop simply rocks. You'll still need to install the back end stuff, which is included in MacTeX.
Linux. If you're running a Debian linux distro, such as Ubuntu, or Linux Mint, it's very easy to get latex up and running. Simply install tex-live from the package repositories, and you're off to the races. Kile is a kitchen sink text editor and viewer that is quite popular.
Cross-platform. TeXstudio is a cross-platform text editor and pdf viewer. It has many nice features, including auto-completion of environments, and the ability to see in real time the equations that are produced from the source code. This is my current platform of choice.
Cloud-based. Overleaf is a nice cloud-based solution. It uses a web-based interface and so it is autmatically cross-platform. As of Summer 2020, with the free "personal" account you may have a total of 1 collaborator on a each project, where you can share results in real time with each other. This service will host all of the files (and images) required to build a project and so there is no need to sync them between different devices. There are other paid options whose price varies. Check their website if you're interested in other choices.
MathB.in is a nice one-off shot at being able to share typeset code.
There are a couple of Chrome apps in development that permit typsetting LaTeX in email form. These are helpful for and sending emails to each other with embedded images of equations.