Communicating with customers, product owners, stakeholders, developer and operations teams a vision for a complete product -- which I tend to call 'the chalupa' because system and application architecture is best communicated in Taco Bell menu items -- is best done visually. Business stakeholders are not programmers. The most effective act of an architect is to deliver an "ah ha!" moment to non-technical people. Also, pictures mean getting out the crayons.
I didn't want to use my big professional design/CAD tool suite for this blog - I use a combination of Visio and Sparx - so I went on a quest for free online tools. I found two I can recommend. They both of are high quality and get the job done.
draw.io -- has essentially the full suite of drawing tools for putting together a diagram: flowcharts, UML 2.0 components (use case, component, activity, sequence diagrams), ERD, gui mockup elements, and BPMN elements. Saves to disk, google drive, or dropbox, exports in a variety of flavors.
lucidchart -- pretty much a full featured design and CAD suite with sharing, group collaboration and Visio imports. Integrates with google drive. Wireframes, mockups, UML, network diagrams, some business modeling. It's neutered, though, to only 60 objects in a design, before needing to go to the pay version.
Of the two, I slightly prefer draw.io. I didn't get working in Lucidchart quite as fast as draw.io and some of the menu items are confusing. They both filled the need to draw something quick on a Surface Pro 2 with only web browser and without a thousand $$s worth of software niche, though. They also both install as Chrome apps.
Go check them out. Blogs need flowcharts!