Pay - if you can - for that password manager, the pro version of your favorite browser, buy some Steam games on Linux. Do what you can to help support the cause you hold dear. Not only are you thanking a developer for their hard work, you’re showing companies that there is, in fact, a market for Linux software.
That’s a good point, paying for software isn’t part of individual Linux users,’ culture. At best, donations are encouraged, but even so you’re not really paying for using the software.
To their credit, Gnome’s software catalog integrates donation links. But you have to scroll to the very bottom to find it, between the project’s homepage and bugtracker links. Giving the donate button more visibility would be an easy thing to do. To my knowledge there’s no store (not gnome nor KDE) that integrate paid apps. Meanwhile iTunes or Steam allow almost one-click buy& install.
I do periodic tax-deductible donations to a local non-profit that help maintaining my Linux distribution. But that’s not much compared to the effort developers put in.