Y Combinator startup accelerator, founded in 2005 by Paul Graham, Jessica Livingston, Trevor Blackwell, and Robert Tappan Morris, has had a significant impact on the startup ecosystem. With a history of successful alumni such as Dropbox, Airbnb, Stripe, and Reddit, Y Combinator offers a unique funding model through SAFE agreements, mentorship, resources, and connections to investors. Its expansion globally and introduction of innovative funding methods have inspired other accelerators and shaped the way startups are supported and funded.