If a program has saved you time, money, or headache, letting the author know is both a good way to keep the program alive and maybe good karma. This brings up the idea of how to pay for something that's free, and not just with money. Additionally, donations sometimes aren't possible due to financial status, international boundaries to money exchange, or the author not wanting to setup a PayPal account.
I came up with some ideas, feedback welcome:
Supporting organizations they support - Mozilla is currently doing this with a wildlife preserve in Hawaii.
Promotion - listing programs that you love on your site
Feedback and suggestions - use with caution as you can easily make it sound like criticism. Should probably be combined with the next one:
Thank the author - contact via email or IM and say thanks.
Feature warrants - Something I recall being discussed on the Pidgin project. I'll give you $X if you add ___.
Disable adblocker on their sites - (if they have adverts) and click on them. [thanks carbonize]
Yeah sometimes it just nice for people to say thank you so you know your works not for nothing. I write a PHP guestbook script which I forked from a now stagnant version and it is just nice if occasionally someone just sends you an email to say thank you. I admit I also have PayPal set up for donations and an Amazon wishlist. It's nice when a package from Amazon just turns up out of the blue.
There is a sixth option and that is to disable your ad blocker on their site and click any adverts that may appeal to you.
