Sloe Gin, traditionally, is sloes steeped in gin with sugar to produce a tart, berry-like winter gin, often sipped as a digestif. Sloes are small stone fruits related to the plum.
While in the UK it's a homemade tradition that peoples' grandmothers are familiar with, in the 1970s and 80s commercial products (mostly used in the Sloe Gin Fizz) were just neutral alcohol with coloring and flavoring.
Plymouth was the first brand I knew of to bring back a real infusion of sloes. 26% ABV. Since then, some other interesting new variations have hit the market.
Greenhook Ginsmiths has a Beach Plum Gin Liqueur, first released in 2012. According to the brand, Beach Plums native type of wild plum that are very close relatives to both damsons and in particular the sloes. Beach Plums are indigenous to New York and the Northeast Atlantic Coast from Maryland to Maine.” The plums are macerated 6-9 months in the gin. 30% ABV
Spirit Works Sloe Gin and Barrel Reserve Sloe Gin: They say "We start with our signature gin, which we macerate with sloe berries until the delicious fruit flavor and color have fully released in the gin." For the Barrel Reserve Sloe Gin, "We begin with Spirit Works Sloe Gin and, much like with our Barrel Gin, let it rest in a full-size new, charred, American White Oak barrel for several months." 29.6% ABV
The Bitter Truth Sloe Gin: "The Bitter Truth Sloe Gin is a Sloe Gin made from the best ripe sloe berries which are macerated in Blue Gin created by the famous Austrian distiller Hans Reisetbauer. Since it is only flavored with fresh sloe berries, this gin reveals the unique aroma of this wild-growing 'little plum', that is widespread in Southern Germany." 28% ABV
Boodles Mulberry Gin: "Inspired by the traditional sloe gin liqueur... the taste combines delicate mulberries with notes of raspberry and currant. " 30% ABV
Averell Damson Gin Liqueur: A sloe gin from damson plums (same as sloes I believe) from Upstate New York.
And, as Wikipedia points out, other plum spirits include the anisey pacharán from Spain, and Japanese umeshu.