As - regardless of PHP 7.0 coming with phenomenal performance and stability improvements - some people refrain from running their own board because older PHP versions had a long history of security quirks, I thought I'd compile a list of actively developed (as in: the latest visible code change should not be older than one year) non-PHP board software with working online demo links for you. Some of them are really neat.
Potentially interesting software without a demo version (try at your own risk):
Is any interesting software missing?

Board Software | Language | Demo board | Notes |
AsmBB | Assembler | Demo | Written in Assembler (!). Does not require any libraries on the server. Assembler! What the actual fuck? |
Atlas | C# (Blazor/WASM) | Demo | (One of?) The first known forum software that runs C# directly in the web browser by making use of WebAssembly. |
cforum | Ruby | Demo | A simple threaded forum, seemingly written for the SELFHTML website (but open source). |
Discourse | Ruby | Demo | Heavily relies on Javascript. Only installation through a Linux Docker container is officially supported. |
DjangoBB | Python (Django framework) | Demo | While the support ("demo") board seems to be dead and full of spammers, the developer is still active. |
FlaskBB | Python (Flask framework) | Demo | The author was inspired by the dropped FluxBB 2.0 preview, FlaskBB's current default theme is basically a copy of FluxBB 2.0's. Quite a simple software yet, e.g. no file attachments (yet?) AFAICS. |
Misago | Python (Django framework) | Demo | One of the most unique boards in this list. The founder tries not to develop "just another forum", a couple of neat interface ideas have already been introduced, more will come. The project website is already "old" - the next version will be much of a rewrite which is obviously a long-term process for one single person. The developer keeps posting development updates though. |
NodeBB | Node.js | Demo | Heavily relies on Javascript. Their website doesn't seem to work well on mobile devices as of now. |
Spirit | Python (Django framework) | Demo | Oh look, another Python-based forum. (SCNR.) Looks like a cleaner Vanilla or NodeBB. The website has an "empty link" named "Pricing", I guess there will be a hosted option available or something. |
Thredded | Ruby (Ruby on Rails) | Demo | A surprisingly fast and simple forum software. |
YaBB | Perl 5 | Demo | One of the oldest non-PHP forum softwares out there. The SMF software had its origins here: "YaBB SE" was a PHP port of YaBB, it was renamed Simple Machines Forum in 2003. They regularly post new beta versions of their software, version 2.7 has been under development for a while. |
YetAnotherForum.NET | C# (ASP.NET) | Demo | Obviously works best on Windows IIS servers. Mono might or might not work (I should test that some day). Can, optionally, look nice. |
Potentially interesting software without a demo version (try at your own risk):
Is any interesting software missing?

Send in the clowns.