Really really good question. I'm looking for something web based that we can work together on because it helps a lot when trying to help each other. Google Docs, which includes Google Draw, is the most collaborative web tool I know today. We can all work on one diagram together at the same time while we chat.
In the meantime, the tools I use (and I do not mean to imply I do any real schematics) include Google Draw (free, cloud, part of Google Docs), GIMP (free, desktop) and flameshot (free screenshot tool for Linux Gnome desktops) for quick markups.
GIMP is the most robust, but complicated and not cloud-based.
I can actually find myself using all 3 for the same drawing within 5 minutes thanks to copy/paste. None of them are designed for schematics, so there is a lot to be desired.
Draw.io is free and web/cloud based. But, not collaborative.
I did a lot of software design diagrams in
Gliffy (commcerial, $96/year, web and cloud). But that is commercial so not likely we can all use it to work together. We had it integrated into our wiki, which made it very collaborative for us. I got everyone in IT and other departments to switch from Visio to Gliffy. Yeah, you could do a few more things with Visio. But the collaboration and having it available to everyone really blew Visio out of the water. Diagrams are communication, and communication and collaboration go hand in hand.
SmartDraw is another commercial cloud one, like Gliffy. $120/year.
Just found
DigiKey. It looks promising and cloud based. As far as I can tell it is free. Does not appear collaborative (don't see a way to invite others to edit your diagram yet). Tied to their catalog, which appears to based on small circuits (fuse holders, but no ANL). Has generic symbols though, like a battery.
The more I play with it, the more I like it. It is customizable. Has plenty of generic symbols. Can create custom symbols.
Here I imported an existing diagram. Then I added at top right a 12V battery and an ANL 300A fuse.