You can support your favorite streamer on Twitch by Cheering using Bits. For the unfamiliar, a cheer is simply a message in chat that uses bits, usually accompanied by an animated emote. Bits are purchased in various amounts, when can then be used one at a time, a few here and there, or all at once.
It is possible to get bits, without spending money, by watching short 30 second ads. Unfortunately, earning bits by interacting with ads is only available for U.S. viewers using a desktop browser.