A tablebase is a database for endings with very few pieces, typically running in conjunction with a chess engine like Fritz, that offers perfect play from the starting position through mate (or to maintain the draw, when applicable). Five-piece tablebases (i.e. both sides' kings + three other units) have been available for something like six or seven years (maybe longer), and some 6-piece tablebases have been available for a while as well.
They can be purchased online at an affordable price (around $50), but for those who wish to sacrifice convenience (and perhaps not only convenience - see below), one can utilize the tablebases for free on
this website. (Hat tip: Alex Herrera)
It's much, much better than nothing, but you are losing out on one thing by using the online freebie approach (in addition to convenience): the link between tablebase and pre-tablebase positions. Sometimes the link will be obvious: you can clearly see that such-and-such a 5- or 6-piece ending can be forced, but on other occasions it won't be anywhere near as obvious. That could prove significant for those of you engaged in postal chess (in events where using engines is legal); otherwise, probably not - unless you're a serious endgame aficianado.