Devpost : Has lots of hackathons by companies, predominantly online. Has Gallery of past projects which cool for inspiration.
HackerEarth: List hackathon challenges, has good balance of online and offline hackathons. MostIndian companies, low prize money. Has competitive programming and hiring challenges to. Post hackathon communication like project gallery does not exists.
GitCoin: This hosts crypto hackathons and bounties. Many hackathons have large prize money but in crypto form.
Devfolio: Has listing for both offline and online hackathons.
Skillenza: Has some online and offline hackathons listings. The offline hackathons are sub par. Mostly focused on students i.e prize money is not worth the time.
Hackathon.io: This is aggregator of hackathons, most of listings are garbage.
eventbrite.com: This is general events website, search by technology or hackathon for some good offline hackathons.
AngelHack: Mostly USA focused hackathons, but they have good prize money.
Kaggle: Machine Learning challenges with good prizes.
TopCoder: Competitive programming challenges, ok prizes.