How did I do this?

This?

That's pixelating, but I'm not sure what tool you might have used to space them.

Gimp was pretty popular back then.I think you used this maybe...:P

pixel spreadsheet?

Maybe pixelation + grid filters in gimp?
EDIT: 1.5x,7px grid and pixelation (not the same values you used, just to give an idea) link

Maybe you used an "image to html" converter? clicky

I've seem some cool converts "back in the days", some let you export as html table, with some you can specify ascii characters for the "hue" of the image.

If you've found a converter that outputs what you want, but you don't have the "grid" you can still edit the html source and set the width to 1 ;)

Edit: even closer to what you want: import the image here

I used a tool called "Rasterizer" or similar many moons ago... used it both as online service or as downloadable .Net or Java executable.

irfanview and pixelize filter?

Thanks for all the comments and help. TheDopefish hit the nail on the head.