With Manning's retirement and Osweiler leaving, the Broncos have one quarterback still on the staff -- Trevor Siemian. He was drafted by the Broncos in the seventh round, 250th overall, in last year's NFL Draft.
As for other options? Adam Schefter is reporting that Robert Lee Griffin III is a possibility.
With Brock Osweiler off to Houston, RGIII is on Broncos' list of replacement quarterbacks, per source. Could be visiting Denver.