Y Combinator font here refers to the font used in the logo of Y Combinator, which is an American seed accelerator company that provides seed money, advice to startup companies.
The font used for the Y Combinator logo is probably FF Meta Normal designed by Erik Spiekermann. The font is a commercial one and you can view more details about it here.
Download FF Meta Font
The FF Meta font identified above is not available for free, please follow the link above and purchase the font. Meanwhile, you can take a look at our collection of fonts for cars, pop music and apparel as well as round-ups of new & fresh fonts around the web.