John, your linux and open source apps are far beyond me. All my accounting clients are on either PC or Mac, so I'm in the pay-for-it (and MS) realm for the most part. Good tips on the WordPress plugins, too!
Shawn, so cool to hear Trello has been so useful to you! I'm with you! I feel I've barely scratched the surface in using it for myself and for other collaborative project management. I've wanted to try mind mapping, though never have, so that's a good tip, too.
I'm going to go out on a limb and detail the agile work apps I've used and why I like some for some things and not others.
free version is single user
I think you have to pay to share notes/reminders/etc.
readily/quickly available offline
syncs automatically between phone and laptop
this is where I keep shopping lists, errand lists, general client notes, other reference items, esp. those I might want on my smartphone while on the go
I've only used the reminders (notifications) a small amount in Evernote - I'm more likely to use Google calendar or Trello
see my post about Trello in the learning to prioritize thread
free version is multi-user, just have to pay for more "bot" or automation features or pretty backgrounds though not necessary for most prioritizing
not as available offline, though usually does okay
has app for desktop and smartphone or tablet
great for lists and project management for groups
this is a more visual, flexible and appealing app than a spreadsheet or other plain-jane document lists
each "card" can be a task - easy to drag around to different lists (such as 'hot list', 'to delegate', 'done!' etc.) or drag up or down on a list to prioritize
plus, each card can have it's own checklist, description, links, images, assignment to one or more parties, etc.
free version is multi-user if each user has enough space in their Google account for all the shared files
in terms of office-type apps, works best with Google Docs, Google Sheets, Google Forms and other Google app files (otherwise the only way to edit something in Drive, like, say, a MS Word doc, is to convert it to a Google Doc file, and now there are two documents...)
in the past, this conversion, and/or the uploading and downloading of documents was confusing to my clients
Google Drive no longer has an app for easier file management on a desktop unless you pay for it
Drive is doing better at being available offline, but was glitchy last I tried (I don't try to access offline much any more, so might be better?)
free and can be multi-user
could mostly be used in browser on desktop/laptop, though available in app for smartphone or tablet, too
can have multiple calendars and the ability to toggle on or off some of them
e.g., Paul's calendar things are green, mine are blue, and wheaton labs calendar items are grey in my set up
many of my regular reminders are scheduled "appointments" and have a notification that requires a dismissal, snooze, or reschedule before moving past it
free with a limited storage amount, otherwise costs ($?)
has decent security for sharing documents in the cloud (well, about as good as any other cloud storage, which is lousy, though perhaps better than sending over e-mail)
this has a file management app for your desktop so that files can be copied or moved to Dropbox as if they are on your computer
this works as storage only - so all types of documents are kept in original format with no need to convert for editing
(which also means each user must have the correct app for editing files on their own device)
if two people change a document at the same time, Dropbox creates a copy with annotated filename for clarity (unless using their new collaborative Paper app/document/thing)
this was far less confusing for my clients and felt more secure than emailing documents with sensitive information
not available offline
login and password app
once you start assisting anyone with anything online, storage of logins and passwords becomes rather important, even paramount
I use Keeper to store passwords for myself, Paul, and about 15-20 clients
Others I know have used KeePass or LastPass
The reason I chose Keeper is because it was device based storage - I could opt out of cloud storage of my sensitive data - and I could sync between my smartphone and my laptop. Though now Keeper insists on syncing through their cloud backups.
Most folks use a browser extension to have the password app fill in the login and password for them (or even simply have the browser remember it for them), but I don't. I don't like the idea of a trojan or hacker having such easy access that way if they invaded my computer. So I copy and paste logins and passwords from my Keeper app into wherever they are needed.