Drawing block diagram?

Free online options

For something quick, there are a growing number of (usually SVG-powered) free online flow chart tools. Here’s a few that don’t require log-in, all pretty basic but user-friendly:

• Draw.io (free and open source) – straightforward, allows saving straight to Google Drive or Dropbox. Also has a Desktop version.
• Gliffy (free up to 5 public diagrams, then pay-for) – clean layout, has a ‘Confluence’ plugin for wikis
• Other answers on this page recommend LucidChart (freemium) and Cacoo (freemium)

For mac / iOS

Omnigraffle is popular for that sort of thing, if what you want is to make connected-block arrow-heavy flow diagrams quickly.

I don’t use it myself, but I know a few information designers who swear by it as a simple way to put together an outline for a diagram, organise their thoughts and get the layout, content and structure right. They then export to Adobe Illustrator to design the presentation of the final product – you could skip this final step if you’re just making simple diagrams like that.

For PC

I’ve heard vague but positive things about Visio, and people sometimes describe Omnigraffle as ‘Visio for Mac’ so I think they’re fairly equivalent.

You can also browse software alternative lists like this one.