What is the most obvious way to tell other people you use Linux. Talk to them!
But what if there are too many people, and you can’t talk to [all of] them really… But still want to get the message "I am a Linux user" delivered? In this case, put this message on something visible. Computer sticker, mug, pen – the options are there. And, of course, with this summer season asking us to change clothes to something light, T-shirt is a nice way to promote your favourite operating system too!
Linux notes from DarkDuck helps you here. Links below lead to merchandise stores of different Linux distributions. Purchasing (and using) items from these stores, you not only promote Linux and tell other people you're a Linux man (or woman), but also help your favourite distribution: part of your purchase goes to the developers and helps them to continue their valuable work.
Also, there are items available on Amazon. Here are the links to Amazon.com and Amazon.co.uk – depending on where you live. Clicking through these links and buying anything there, you help this blog.