A Hori-Hori, sometimes referred to as a "soil knife" or a "weeding knife", is a heavy serrated multi-purpose steel blade for gardening jobs such as digging or cutting. The blade is sharp on both sides and comes to a semi-sharp point at the end.
FUN FACT: The word “Hori” (ホリ) means "to dig" in Japanese and "hori-hori" is the onomatopoeia for a digging sound. The tool itself is commonly referred to as a レジャーナイフ, "leisure knife" or a 山菜ナイフ, "Sansai (Mountain-vegetable) knife" in Japan. (from Wikipedia)
My favorite little “dig-dig” tool can saw through soil, roots and small twigs and branches. Its curved blade can be used as a trowel to take care of most of your basic needs in the garden.