Draftsight is free, or cheap, depending on whether you get the free or professional versions.
Is close enough to being AutoCAD that skill with either is reasonably interchangeable. The basic drafting method- board drafting on a computer is the same.
Draftsight works the AutoCAD .dwg file format, as near to a universal engineering drawing format as you can get.
Draftsight has the essential functionality of AutoCAD, vector graphics, layers, blocks, associative dimensions and annotation.
Draftsight is available for download and has a small system footprint and is always there in a pinch.
There are command line aliases, so when you type 'REFEDIT' at the command line, you get REFEDIT, even though it's called something else in draftsight.
User support via google or the draftsight community is available, but not as universal as AutoCAD solutions on the internet.
It's close enough that the differences are confusing and frustrating.
The pointer tends to lag and be otherwise not the same as AutoCAD. A small but annoying difference.
You can mess up drawings made with AutoCAD Mechanical, MEP or other specialized versions by reducing special annotations, bubbles, bills of material, weld symbols, etc. to simple vectors, which will display properly but can't be edited in the same way they were before. This is something to watch out for and can make revising those same annotations a pain, or more easily redone than troubled with. This is something to be aware of with .dwg files in general.
Draftsight does not have 3D solid modelling, but I haven't used that for years, no big loss for me.
While Draftsight is not a full seat of AutoCAD, not everyone needs it or can afford it, this is where the price of free or cheap comes in handy. You can put it in front of an engineer, temp or other person you don't have drafting on a regular basis and they can make a scale sketch or drawing for you that you'll be able to plot directly, or pull into almost any CAD application as basic geometry. Remember to mind the basic units when you import!
Basic, nothing fancy drafting & design. Working with other people that work with AutoCAD.
Adding temporary help without buying more software.
Occasional use of .dwg files that does not merit the cost of an AutoCAD license- the occasional electrical schematic .dwg in a shop that otherwise works in Solidworks for instance.