Factoid:
In Brain-Flak a one is shorter than zero and multiplying by seven takes more characters than multiplying by eight.
Since Brain-Flak programs must have an even number of characters I have added some programs with extra spaces for the odd length snippets. These are not well golfed intentionally but rather put there in an attempt to give you a little more for your kind upvotes.
###Length 2 snippet
<>
It switches to the offstack. As a single program this will always output nothing regardless of the inputs.
###Length 3 snippet
{ }
This is the {} nilad it pops top item on the stack and returns its value. As a full program it just removes the last item input.
###Length 4 snippet
(())
This program pushes one to the top of the stack. This is the shortest way to express 1 in Brain-Flak.
###Length 5 snippet
([ ])
This program pushes the stack height to the top of the stack. At the start of a program it acts like argc.
###Length 6 snippet
({}{})
This program adds two numbers.
It pops the top two elements and pushes the sum.
###Length 7 snippet
This one requires a -d flag to run so +3 bytes
(This one also doesn't work on try it online)
@ij
This is the injection flag! (With a space after it because it needs to be length 7)
This halts the program takes a Brain-Flak program from STDIN and runs it as part of the code. Its my favorite flag and (arguably) Brain-Flak's shortest self interpreter. @ij flags can be nested