Chartboost (Raised 21m from Sequoia Capital, et al.) and SendGrid (Raised 27m from BVP, et al.) are probably the two largest (by valuation) startups which have

Chartboost (Raised 21m from Sequoia Capital, et al.) and SendGrid (Raised 27m from BVP, et al.) are probably the two largest (by valuation) startups which have publicly stated that they were rejected from YC. There may be others of similar size who haven't publicly disclosed, but it's unlikely there's anyone an order of magnitude larger (pg has stated there hasn't been any YC reject who has reached the $1bn+ valuation mark). Only eight YC companies have raised more than $20m in investment (dropbox, airbnb, xobni, stripe, loopt, scribd, hipmunk, and optimizly). LightSail Energy ($48m; Kholsa, et al) was also technically a YC reject but given how far it is outside of "typical" YC companies I'm not sure I'd count it.