Not OP, but: I have sunk a few hundred bucks into Steam after it added Linux support. I had a Steam account before that, but I never purchased anything that cost money.
As with others here, various JetBrains tools and Linux-compatible Steam games (I have 75 and counting). Also a file comparison tool called Beyond Compare.
If Adobe released their apps on Linux (more specifically Photoshop and Premiere) I'd buy them in a heartbeat. Unfortunately they seem more interested in supporting the iPad if recent news is to be believed.
Edit: Just remembered I have a Sublime Text license too, although I stopped using it a while ago.
A couple of months of Intellij because a customer wanted me to work on a Java project which used that IDE. I delivered in the first month and paid for the second one because I didn't realize it was autorenewing. That was a long time ago.
I also received a Steam game from a friend as Christmas present. I didn't pay but he did.
I used to get my distributions from Walnut Creek, then bought a few boxed editions of Mandrake and SuSE, for several years subscribed to Linux Journal since the yearly days, occasionally bought Linux magazines with CDs on them, did FSF donations a couple of times, nowadays every time I download Ubuntu I always donate donations.
Likewise on Windows apps that I use regularly like Notepad++, jAlbum, Sublime Text, Paint.NET, Thunderbird, Firefox...
I'm also interested in this question. Because looking at myself, I have paid for several apps on both Windows and MacOS but never on Linux. Although to be fair I stopped using Linux as a desktop OS before I had the economic security needed to purchase apps in the first place.