To look at the flip side, it is rare for YC to invest in companies that did not actually go through YC (it has happened before, however).
YC has invested in 2 companies that went thru 500 accelerator first (9GAG and Vayable), and also a few others where we invested at around the same time as YC (Scoutzie, Virool, Microryza, Referly).
also, we are investors in ~20 TechStars companies (including Sendgrid), and also ~10 companies each from AngelPad in SF, and SeedCamp in Europe, and StartMate in Australia.
altho we run our own accelerator program at 500, we are frequent investors in other accelerator programs, including several around the world in East Asia, South Asia, Latin America, Western & Eastern Europe, and the Middle East.