StrangerMeetup and Omegle are two popular services that let you chat online with strangers. Although both services work on Web browsers, Omegle is also available via third-party applications for iOS and Android.
Even though both services offer anonymous chat, StrangerMeetup also allows the user to create an account to access more features. Using Omegle, you can start a completely random conversation, state your interests to talk to like-minded people or ask a question and watch two strangers discuss your question.
As both chat services are available to practically anyone, you may encounter adult or offensive content when using them. Exercise your best judgement when using the services.