Chenopodium album, the white goosefoot.
Other related species, such as Good King Henry (Blitum bonus-henricus)
Chenopodium album, white goosefoot
Polygonum aviculare (common knotgrass)
Amaranthus species
Portulaca bicolor, of Australia
Heracleum sphondylium
Any of various weedy plants sometimes used as pig fodder