Zig-zag pattern 3 #2416
 Grid  CC 0

A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.

Source Firkin

 More Textures
Darth Stripe #99
 Dark  CC BY-SA 3.0

Diagonal lines with a lot of texture to them.

Source Ashton

Prismatic Polka Dots Mark II No Background #530
 Noise  CC 0

Prismatic Polka Dots Mark II No Background

Source GDJ

Medic Packaging Foil@2X #377
 Light  CC BY-SA 3.0

8 by 8 pixels, and just what the title says.

Source pixilated

Background pattern 208 (colour 2) #2462
 Grid  CC 0

A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.

Source Firkin

WaveCut@2X #76
 Light  CC BY-SA 3.0

If you like it a bit trippy, this wave pattern might be for you.

Source Ian Soper

Colorful Floral Background 3 No Black@2X #486
 Light  CC 0

Colorful Floral Background 3 No Black

Source GDJ

Paisley Background #495
 Light  CC 0

Paisley Background

Source GDJ

Background pattern 215 (colour 4) #2368
 Green  CC 0

A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.

Source Firkin

Part of Bayeux Tapestry 4 #2446
 Noise  CC 0

From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.

Source Firkin

Fish Pattern Background #233
 Noise  CC 0

From PDP.

Source GDJ