Unicode character for PUZZLE PIECE?

The Unicode Consortium doesn't list a puzzle character.

And really, even if it did, I'm not sure you should use it. It relies on that glyph being available in the font you're distributing, and that's far from a sure thing, especially for the more obscure characters.

This is, for me, the canonical "plugin icon":

And I can think of two routes to go:

• If you're sold on having it be a character you can create your own font, using Fontlab or something similar, then distribute the font along with your app. It turns out to be easier than it might seem. This has the advantage that you can treat the "plugin" picture just as another piece of text.

• Better may be to just use an graphic. Including a graphic is easier, and works just as well if you're trying to use it for an icon or button.

I don't think there is a character like this. At least, I've never seen one.

Note that if you are using Windows, you can just type charmap.exe, select an Unicode-compatible font, and view every available character. Probably similar tools are available for other operating systems too.

What else would you use as symbol for application plugins?

Depending on the context, you can use different things. A plus + symbol may be the one which symbolizes probably the better the idea of a plug-in (IMHO even better than a piece of puzzle, since a plug-in is something which adds a feature to software, not a part which must be combined with other parts to form a whole application).

Why not using an icon? Keep in mind that using a fancy unicode character may cause some problems with non-unicode-compatible software, browsers and environments.

Unicode 11.0, released June 5, 2018, added a character for a puzzle piece: U+1F9E9.

See the Emoji page for a visual reference.

Software support isn't so great at the moment as this depends on updated fonts. But most social networks do support it already.