Japt, 3 bytes

o ï

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C (gcc), 83 51 bytes

Saved 32 bytes thanks to Kevin Cruijssen.

i;f(n){for(i=0;i<n*n;)printf("%d,%dn",i++/n,i%n);}

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I'm by no means a C programmer (or C golfer), but I thought I'd give it a try. Should the main be included in the byte count?

R, 33 25 bytes

which(diag(scan())|1,T)-1

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Thanks to Kirill L. for suggesting a 2-byte golf, which inspired me to look further :-)

PowerShell, 48 42 bytes

param($n)0..--$n|%{$i=$_;0..$n|%{"$i,$_"}}

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Boring double-for loop.

Saved 6 bytes thanks to mazzy.

05AB1E, 8 bytes

L<ãí',ý»

Exactly as the challenge description: prints the 0-indexed coordinates ordered by y-then-x comma- and newline-delimited to STDOUT.

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Explanation:

L          # Create a list in the range [1, (implicit) input]

           #  i.e. 3 → [1,2,3]

 <         # Decrease each by 1 to make the range [0, input)

           #  i.e. [1,2,3] → [0,1,2]

  ã        # Create each possible pair with itself

           #  i.e. [0,1,2] → [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2],[2,0],[2,1],[2,2]]

   í       # Reverse each pair so they're sorted by y-then-x instead of x-then-y

           #  i.e. [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2],[2,0],[2,1],[2,2]]

           #   → [[0,0],[1,0],[2,0],[0,1],[1,1],[2,1],[0,2],[1,2],[2,2]]

    ',ý   '# Join each pair with a space delimiter

           #  i.e. [[0,0],[1,0],[2,0],[0,1],[1,1],[2,1],[0,2],[1,2],[2,2]]

           #   → ["0,0","1,0","2,0","0,1","1,1","2,1","0,2","1,2","2,2"]

       »   # And then join everything with a newline delimiter (and output implicitly)

           #  i.e. ["0,0","1,0","2,0","0,1","1,1","2,1","0,2","1,2","2,2"]

           #   → "0,0n1,0n2,0n0,1n1,1n2,1n0,2n1,2n2,2"

05AB1E, 2 bytes

Lã

Returns a list of 1-indexed coordinates ordered by x-then-y.

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Explanation:

L     # Create a list in the range [1, (implicit) input]

      #  i.e. 3 → [1,2,3]

 ã    # Create each possible pair with itself (and output implicitly)

      #  i.e. [1,2,3] → [[1,1],[1,2],[1,3],[2,1],[2,2],[2,3],[3,1],[3,2],[3,3]]

Tcl, 70 bytes

proc C {n i 0} {time {set j 0

time {puts $i,$j

incr j} $n

incr i} $n}

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C#, 59, 65

First time posting. Apologies if I do something wrong!

l=>{for(int j=0;j<l*l;)System.Console.Write($"{j%l},{j++/l}n");}

Python 2, 39 bytes

lambda n:[(i%n,i/n)for i in range(n*n)]

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Pyth, 4 bytes

^UQ2

Full program. Outputs list of coordinate pairs.

^UQ2   Implicit: Q=eval(input())

 UQ    [0-Q)

^  2   Take the cartesian product of the previous result with itself

MathGolf, 2 bytes

r■

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Explanation

r    Range(0, n)

 ■   Cartesian product with self for lists

For pretty-printing, you could add n to have it print one list item per line.

APL+WIN, 11 bytes

(⍳n)∘.,⍳n←⎕

Index origin = 0. Prompts for input for n and outputs the following for n=4:

0 0  0 1  0 2  0 3

1 0  1 1  1 2  1 3

2 0  2 1  2 2  2 3

3 0  3 1  3 2  3 3

Lua, 63 bytes

s=io.read()-1 for i=0,s do for j=0,s do print(i..','..j)end end

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Pepe, 88 bytes

I've never done 2D iterating in Pepe before and it doesn't seem to work pretty well due to labels being dynamic. There's quite a lot of two byte commands to avoid moving the pointer.

REREeErEErerErEReREErEEEErreEEreeeEeEEeerEEeerreEErEEEEEreeEReererEEEEErERRREEEEEeRrEree

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Warning: Do not run it with input below 1 - it will kill your browser.

Perl 5 -na, 35 bytes

map{//;say"$',$_"for 0..$F[0]}0..$_

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Powershell, 43 bytes

param($n)$i--..--$n*++$n|%{$i+=!$_;"$i,$_"}

Explanation:

One row 0..$n-1 repeated $n times.

Pure Bash (no external utilities), 35

eval echo {0..$[$1-1]},{0..$[$1-1]}

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Perl5, 42 38 bytes

for$i(0..--$n){for(0..$n){say"$i,$_"}}

(The inner loop saves a few bytes by using the implicit variable "$_" as an index.)