For Windows users:
The problem with the solution accepted here, in my opinion is that if you already have Chrome open and try to run the
chrome.exe --disable-web-security command it won't work.
However, when researching this, I came across a post on Super User, Is it possible to run Chrome with and without web security at the same time?.
Basically, you need to add to the command and run it like this instead (or create a shortcut with it and run a new Chrome instance through that)
chrome.exe --user-data-dir="C:/Chrome dev session" --disable-web-security
which will open a new "insecure" instance of Chrome at the same time as you keep your other "secure" browser instances open and working as normal.
This works by creating a new folder/directory "Chrome dev session" under C: and tells this new Chrome instance to use that folder/directory for its user and session data. Because of this, the new instance is separated from your "normal" Chrome data and your bookmarks and other saved data will not be available in this instance.
Note: only the first "new" instance of Chrome opened with this method, is effected, hence it is only the first tab in the first new Chrome window, which is effected. If you close that instance, you can use the same command again and for example any bookmarks to your local app or similar will still be there as it's pointing to the same folder.
If you want to run multiple "insecure" instances, each one will need its own folder/directory, so you will need to runt he command again with a different folder name. This however also means that each insecure instance will be separated from the others, so any bookmarks or other saves user or session data will not be available across instances.