Basically: one of those little boxes on your site that lets people send you an email (or instant chat) directly from there, without opening their email client.
Value: I mainly wanted this so that people could warn me more easily if there were issues with the site, if they were unable to log in, if something was broken or they were confused, etc. In practice this has happened a little, and other people have written just general messages and comments from subscribers that they probably would have sent by email otherwise, but perhaps the lower friction of sending direct from the website helped a little. We ended up getting 5-10 messages per week.
Issues: Main issue was that these are designed for super aggressive Conversational Marketing, so by default they all seem to pop up in users' faces repeatedly or send out random bot messages every few minutes. But once I disabled all those options it was just a discrete "chat button" in the bottom corner that doesn't open unless the user clicks it, and then just shows a form where they can submit a message (see screenshots below).
Most of the widgets are also designed for Live Chat rather than email, but I don't have the time/patience/masochism to staff that, so I just permanently stay in Offline Mode so that any messages will get forwarded to my email so I can reply directly from there.
Brands: there's a horrific number of companies offering this service now -- Intercom and Drift are prominent but every site I go to these days seems to have a different widget, all mainly targeted at high-touch sales and b2b products. I ended up using https://www.tawk.to/ because it was free and seemed the least-overkilly (nb their default styling is insanely childish, but you can change it). Maybe there's a better option out there that literally just lets people email you direct from the site and nothing else, if you see one please do let me know.
Difficulty: It's a very simple code injection header to set up on your site, on Ghost I just did it myself, on Substack I emailed them and asked them to insert it for me. For the widget modification, Tawk.to's interface was kind of dumb and confusing at first, main issues was figuring out how to turn off all the features I didn't want.
Price: tawk.to is free, or you can pay $12 per month to remove the tawk.to branding on the widget, but it's pretty discrete anyway (screenshot below)
Risks etc: possible that the widget is slowing down the site? I haven't bothered to check yet.