Gallup polls individuals about various topics and reports what they find. Many of their polls are public opinion polls. A few are listed below. Gallup's main site is the top link.
Alongside other research, the Pew Research Center uses public opinion polls to glean data on "the issues, attitudes and trends shaping America and the world" ("About" 2016). The link below is Pew Research Center's homepage.
These are other polling groups or organizations.