A spider, Missulena occatoria (Australia).
Myth: You can always tell a spider because it has eight legs.
Fact: Not exactly. Scorpions, harvestmen, ticks, and in fact all arachnids—not just spiders—have four pairs of legs (see illustrations). Insects have three pairs. Also, notice that I said "four pairs" instead of "eight." The number of leg pairs (one pair per leg-bearing segment) is more significant than individual legs, which can be lost.
Examples of 6 of the 11 orders of arachnids. All have eight legs. Only one is a spider.
A whipscorpion, Abaliella dicranotarsalis.
A scorpion, Charmus indicus (India).
A pseudoscorpion, Chelifer tuberculatus (Algeria).
A harvestman, Phalangium opilio (worldwide).
A solpugid, Ammotrechella cubae (Cuba).