The majority of Gates wealth is clearly in Microsoft stock, but he also has several investments in other companies.
According to some sources, Gates's personal investments in biotech alone are worth between $300m to $400m. He is also known to have stakes in satellite TV companies along with media moguls like Rupert Murdoch. Don't underestimate the power of Bill.
Of interest: He invested in a company called Icos in 1990, a company that developed an erectile dysfunction drug. Hmmmm, do I need to say anything?