It is difficult to maintain any project, open-source or proprietary. There are real challenges with each. The issue of revenue is something that independent contractors, small and large companies/organizations must deal with on a daily basis. It is about survival and longevity. I also think it is about legacy as well. How does the project go on? Does the project have the resources to sustain itself and the developers? Can the project still maintain its momentum? Not everyone have access to the financial resources or the connection of Shuttleworth or Brin.

This brings me back to Vladimir’s original question and I will use the example of a real ice cream company that updated their customers Twitter every time they bought an ice cream. I read this article about an ice cream company was trying to use social media to raise branding and profits. They company launched a campaign to encourage customers to link their Twitter account to their account on the ice cream company. Customers who signed up for the campaign provided their Twitter login, which is entered into the ice cream system. Every time the customer buys an ice cream, their Twitter page will be updated with what they purchase. So, if the customer buys vanilla ice cream, the tweet will read something like: Bought vanilla ice cream on June 9 at one of the stores of the ice cream company. What is interesting about this example is that the company did not and is not waiting for the customer to update their Twitter. Instead, the company re-engineered their POS system (cash register) to send the tweet automatically when the customer makes a purchase. The company is trying to reach all their customer’s followers as potential customers. Imagine if the person eats their ice cream 4 times a week, the followers of that customer may decide to try that ice cream.

Coming back to your question, you can continue to make all your plugins freely available and supporting them as you already do. Or, you may decide to charge a fee for some of the plugin or for all. You can also provide paid support, which you may or may not be doing, to anyone that needs immediate assistance for your plugin. This model allows serious and interested people to purchase the service, and for others to use the forum to solicit for assistance.

You cannot make everyone happy, even if you do every thing right. I also see no issue with adding the link to your website and books to the options, or even making the link to your website present on the user website. The user can always edit this information out if they choose, or leave it to show supports. Finally, people that use WordPress may not necessarily be interested in making a plugin. You have to remember that there are users that are simply looking for a plugin that work and help their website. Instead of targeting the book of how to make a plugin to them, you should target your service instead.

Take a look at this article about software pricing:
http://www.inc.com/magazine/20101101/go-ahead-raise-your-businesss-prices.html

There are many ways to support authors who are publishing free plugins. If you cannot donate, there are other ways to contribute. My recommendation is as follows:

1. Blog about the plugin
If you like the plugin, let others know about it and share your experience. Submit your experience to the forum, WordPress, etc.

2. Get others to donate
Happiness is infectious. When you are happy about something, chances are someone else will become happy as a result of your happiness. When someone is happy, they can do some happy things such as donate. There are people who are looking for avenue to donate. Why not a plugin?

3. If Google ads are a must for the plugin authors, give administrators the option to activate and deactivate the plugin, and its appearance on the website.
For example, you can have the plugin show on the website, selected pages, all the pages, or none of the pages.

The truth is that someone has spent time developing and maturing the software. If it is something that is useful to you, support it. If you cannot support it immediately, do your part and let others know. It is the least we can all do.

This is Joomla's blog in which they're considering the future of their financial model. They analyzed other open source companies to help guide their own. About a page down, there's a very brief chart of how each company stays afloat:
http://community.joomla.org/blogs/community/1117-how-open-source-development-is-funded.html

As long as the obnoxious ads are avoided (which you aren't even close to) I think it's very well appropriate to put links like that in your options menu. I have found it on many others. It's like signing a painting or any other work. Because of how aware you are, the only possible way I could think of being even more sensitive is to mention it in the plugin description. Something at the bottom like, "Much time and energy has been invested in this plugin. So naturally, in the plugin options I give links to support, website, and other plugins. As it relates I also mention my book about developing plugins. By downloading the plugin you agree to appreciate the developers work, and not complain about the unobtrusive links. If you disapprove, please refrain from downloading."

Now that I wrote that, I think it's still going overboard. I wish I could, but given my situation, I don't feel the freedom to donate, so any other way I can give back, I attempt to do so. When I get my site up and running, I'll have a link to everything that powers the site, mentioning your plugins and blog. It's the least I can do.

All the best!

In any open-source project, you need revenue to support the momentum, and to ensure the project sustainability. I have seen a lot of projects that started off with lots of enthusiasm, but succumb to a premature death. This occurs too often.

I think that developers and designers need to rethink their own strategy for viability. Tricky implementation and shady undercover dealings that work in boosting their websites at the expense of their users will not work.

Gone are the days of free. The web is increasingly becoming service oriented. I personally would pay for a plug-in if it is active development, if the developers listen to their users, and if there are timely releases to address user issues as well as security.

Vladimir, I think you have to think about your sustainability. The popular model of donation, or as I call it, solicitations, in Open Source is not always a viable option. It is sort of like those Google Ads. You see them, you know you are supposed to click on them to help the website owner generate revenue, but many people do not.

In an ideal world, donation is really a selfless act, but we don't live in one. Perhaps, we should start thinking about cultivating one.

As a web design firm, you know that I'm going to go right to the plugin author when I need that plugin modified a bit. I would put a link to donate and a link to hire you for custom mods in every plugin.

You will always get folks that will object, whatever you do I guess, but I would be surprised if almost everyone that downloads your plugins aren't bith grateful and delighted with your work.