It's pretty common, on IRC, for someone to say, "Thanks a lot, I owe you a beer." I say it a lot, too, but I'm not sure I've ever actually bought someone the promised beer. I don't feel too bad about that: I'm not sure anyone has bought me such a beer, either. Sometimes, "I owe you one" just means "I appreciate your help."
Then again, a few times during my time as a free software author, users have surprised me with actual gifts. In almost every case, they've gone to my published wish lists and bought me a little something. Recently, I got an email like this:
Subject: you have a gift! Chauncey Gardner has sent you a Kindle edition of "How To Eat Pie Faster" Personal Message: Thanks for your work on Games::Nintendo::Mario! It has saved me a lot of time at work!
This is delightful. It could be a $1 Kindle Single or a 99 cent iOS app and I would still be utterly tickled that someone found my work so useful as to send me something nice.
Surely I am not the only person who feels this way. I bet plenty of other people would be equally delighted, even if they don't know it. This realization has put me on a mission to give a lot more gifts. I think you should, too -- and you should make it easy for people to do the same.
Make a wish list. Put it somewhere that interested parties can find it. Put a lot of small things on it. A $2 gift is a real treat, and if you're happy with it then the giver has plenty left over for the next guy whose software has made his day.
Make sure you're using a wish list where purchased things are marked as purchased -- or where you don't care if you get duplicates.
Avoid gift certificates. That way, the giver is giving you a real thing, and you can thank him with a personal note about the item: "Wow, thanks! I've been meaning to pick up this book for years, but it never got to the top of the list. Maybe this year I'll finally beat Jimmy J. at the Eastern Seaboard Pie Eating Contest. Here's a picture of me accepting the silver trophy last year..."
By the way, see what that means? It means that when you give somebody something he or she wants, you might get to see a picture of your favorite software's author covered in the remains of 23 blueberry pies. Even if not, you will have learned something about the person by reading the wish list, and get to feel giddily excited as you click "checkout" and know that the person who has delighted you is about to be delighted in turn.
If you do list gift certificates, tell people what you spent it on. It makes it clear that you accepted a gift and not a donation. A $5 donation is not going to help most people pay their bills much better. An unexpected $5 gift, though, will be a real high point in their week.
Make a list of people whose work has really made your life easier. Work through it at your leisure. If they don't have a wish list, don't skip them. Write an email. There have been several occasions where I got to work in a rotten mood, started to glower at a bug list, and then noticed a simple email of thanks for some patch I applied or advice I gave. These have always, in every case, turned my day around.
If you get to the end of your list, find out why. Most free software isn't just one programmer. Go look at the project's commit logs. Who fixed that bug in the last release? It might not have been the project's public face. Was it some guy who wrote a one-line patch and never contributed before or after? Who cares, he solved your problem! Thank him profusely.
Finally, if you think you can only give and not receive, because you have never contributed to free software, I am guessing that you are wrong. If you show up on an IRC channel or mailing list and ask a question that leads to the project getting better documentation, you are a big help. If you submit a bug report that leads to the bug being fixed, you are a big help. If you're not doing anything at all like this, then the best gift you can give the rest of the free software community is to start.
...as a footnote, if you're looking for a CPAN author, and that author has put a wishlist link in his or her metacpan author profile, you can look at the author's page on metacpan. You can even look for everyone who has a wishlist URL with a carefully crafted metacpan query