If you've been meaning to get in touch with someone for a while but just haven't got around to it, do it now.
Seriously, you don't know how much of an effect just getting in touch can have. You don't have to say much. Just a simple indication that they're being thought about can change someone's day. It has brought me out of the depths of despair on more occasions than I can count.
Try it again tomorrow, too. And then the day after. And the day after that.
The specific focus of his year's Mental Health Awareness Day is mental health in the workplace. There's lots of advice on twitter today, but if you want one more thing to do to help your colleagues, then try this:
Ask a colleague how they are, and if you can help with anything. Mean it.
Last thing, I promise: don't try to aim the above just at people that you can identify as having mental health problems. Most people struggling on the inside hide it pretty well, and little if anything makes it out to the surface. If you think you don't work with anyone who has a mental health issue, it's almost guaranteed that you're wrong. Whoever you are, whoever you work with:
Go out of your way to be explicitly and repeatedly nice to people.
This strategy has absolutely no negative consequences and many positive ones.