Not all my friends are super-technical geeks that are spending their life in a terminal, but most of them are using computers on a daily basis, often with the sempiternal same suboptimal tools and practices, especially when it comes to collaboration and project management.
There is of course a myriad of different tools available on the internet, but this list will only contain libre ones, for a lot of reasons, one of them being that you may want to switch to other services, or to deploy them on your own infrastructure instead of trusting the cloud.
The goal is obviously not to list every libre software that you
should could use, but only the main ones that everyone should at least know about.
Please keep in mind that when you're using third-party services (aka the cloud), you're trusting them to avoid doing nasty things with your data, so don't be silly.
For real-time notes taking (like during a course), a cool solution could be to use a pad, an online text editor providing collaborative edition, in real-time. Etherpad is the most famous implementation and has a lot of public instances available.
If you don't care about collaborative edition, there is mindmaps, that can run offline, and is completely client-side.
Instead of using Skype, you can use irc, with clients like pidgin, hexchat, … You can connect on any servers you want, freenode, oftc, … pick the one you prefer, connect to it, create a room, and that's it.
Don't forget to combine this with meld if you work with emails ;)
Instead of using Trello, you can use Wekan, an open-source and collaborative kanban board application, that you can use on its public instance. You can take a look at its roadmap if you want to see how it looks like.
Of course, don't do that if your pictures are private.
I wrote this blogpost mainly for linking it to my friends, but I hope that it will serve you too. Feel free to email me other alternatives if you think that I missed some.