QBasic, 6 3 bytes

CLS

Clears the screen.

Try it at Archive.org!

If it turns out there is technically some way to do this in BF (e.g. by outputting a bunch of newlines), then here's a 5-byte solution:

FILES

Outputs the contents of the current working directory.

Batch, 2 bytes

hh

Launches HTML help (which displays its own window).

Google Sheets / Excel, 6 bytes

=NOW()

Returns the current date and time.

PARI/GP, 2 bytes

\s

Attempt This Online!

\s prints the state of the PARI stack and heap, which is non-deterministic.

Example output:

 Top : 7fadf82bd000   Bottom : 7fadf7b1be00   Current stack : 7fadf82bcfb8
 Used :                         9  long words  (0 K)
 Available :                    999991  long words  (7808 K)
 Occupation of the PARI stack :   0.00 percent
 1 objects on heap occupy 9 long words

 10 variable names used (10 user + 0 private) out of 65535

Copied from my answer to another challenge.

Python 3, 11 bytes

open(*'ww')

Create a file called w in the current directory.

Funge-98, 4 bytes

6y,@

Try it online!

Outputs the operating system's path separator (/ for POSIX, \ for Windows).

C++, 46 bytes

#include<ios>
main(){freopen("x","w",stdout);}

Creates a file called 'x' in the current directory (if x does not exist). I think this still counts.

Try it online!

JavaScript (Node.js), 9 bytes

_=>Date()

Try it online!

Ruby, 3 bytes

Prints the command-line arguments.

p$*

Attempt This Online!

Bash, 2 bytes

>_

Try it online!

Create a file named '_' in the current directory.

APL+WIN, 3 bytes

Outputs the current time to the nearest msec.

⎕ts

Try it online! Thang to Dyalog Classic

Trivial answers that output command-line arguments

Many languages can easily read command-line args:

Pip, 1 byte

a

Outputs the first command-line argument. Attempt This Online!

Jelly, 0 bytes

A full program that prints 0 unless provided an argument in which case it prints^* the Jellification^** of that argument evaluated as Python code if possible or the raw argument as a string if not. (Satisfies the requirements since Brainfuck can only read from stdin, not from command line arguments).

Try it online!

^{* an empty lists prints as nothing; a mixed list (characters and numbers) smashes its element's representations together, a singleton list prints its element.}

^{** None becomes an empty list; complex numbers with no imaginary part become floats; dictionaries become a list of their keys if all keys are strings; probably other things may happen in other exceptional cases}

Python 2, 11 bytes

print id(0)

This program outputs the address of the object 0 in memory.

Run it here

Javascript, 7 bytes

alert()

Window is the point