Ways to do live polling (aka ConcepTests) during class
My students are all on a Discord server with me, so when it comes time for polling, I hijack the reaction system in Discord to get it done.
One nice feature is that it does syntax highlighting for any language that you specify, and you can also install mathjax on your server if you'd like to be able to represent mathematical functions.  Here's a question I polled recently.  The question itself is quite a lot of fun, and is from Java Puzzlers by Bloch and Gafter:
**What will this Java code output?**

```java
public static void main(String[] args){
   final String pig = &quot;length: 10&quot;;
   final String dog = &quot;length: &quot; + pig.length();
   System.out.println(&quot;Animals are equal: &quot; + pig == dog);
}
```

React with:
:one: `Animals are equal: true`
:two: `Animals are equal: false`
:three: `false`
:four: The code will fail on an exception

This outputs in Discord as:

Students then react with one, two, three, or four.1
It doesn't allow reactions to be hidden, so if this is an important feature to you, then I would look elsewhere.
I've also found it useful for getting a sense of where my students are in an activity when they go to breakout rooms to work in pairs or trios:

This is very informal polling, but you can see how useful such information can be during virtual instruction.
1 - The answer is 3.  Surprised?