The mining pool ghash.io briefly exceeding 50% of the bitcoin network's computing power in July 2014, leading the pool to voluntarily commit to reducing its share of the network. It said in a statement that it would not reach 40% of the total mining power in the future.
Krypton and Shift
Krypton and Shift, two blockchains based on ethereum, suffered 51% attacks in August 2016.
In May of 2018, Bitcoin Gold, at the time the 26th-largest cryptocurrency, suffered a 51% attack. The malicious actor or actors controlled a vast amount of Bitcoin Gold's hash power such that even with Bitcoin Gold repeatedly attempting to raise the exchange thresholds, the attackers were able to double-spend for several days, eventually stealing more than $18 million worth of Bitcoin Gold.
The tangle, a distributed ledger that is fundamentally distinct from a blockchain but designed to accomplish similar goals, could theoretically succumb to an attacker deploying over a third of the network's hashrate, referred to as a 34% attack.