A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
Prismatic Hexagonalist Pattern No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Prismatic Groovy Concentric Background No Black
Source GDJ
Have you wondered about how it feels to be buried alive? Here is the pattern for it.
Source Hendrik Lammers
Zero CC Mossy stone tileable texture, photographed and made by me. CC0
Source Sojan Janso
Floral patterns will never go out of style, so enjoy this one.
Source Lasma
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Zero CC tileable hard cover cells, skin like, book texture. 4K, Scanned and made by me CC0
Source Sojan Janso
A free green background pattern with a pattern of rhombuses on a seamless texture. Feel free to use it as a tiled background image on your web site.
Source V. Hartikainen
Seamless tile drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
Prismatic Snowflakes Pattern No Background
Source GDJ
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
A seamless web background with texture of aged grid paper.
Source V. Hartikainen
Prismatic Hypnotic Pattern 2 No Background
Source GDJ
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
A bit like smudged paint or some sort of steel, here is scribble light.
Source Tegan Male
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
Seamless SVG vector and JPG backgrounds with faded diagonal stripes. The colors are editable.
Source V. Hartikainen