Most open source software is built completely by volunteers (without compensation) because they want a product that fits their needs. The list of contributors may seem large, but every person has different skills to offer and your skills may be what it takes for a project to evolve. In most cases you do not need to be a programmer to help, many projects need graphic artists, document writers, webmasters, database administrators, etc.., if you wish to offer your time to help a project you should check their home page for a listing of needed skills or you can write their mailing lists to ask if any of your skills can be utilized.
By adding your hands, eyes and associated skills to their project you may make a valuable contirbution that can push the project and their competitors to advance technology to the next level, so please do not be afraid to ask and offer your time.
Time is money and the time that people spend working on a project may cost them money from paid work they are missing and in some cases certian projects have paid full-time staff to guarantee their rate of evolution. Feel free to visit the project's home page to see if a donation link is supplied to donate what you can or what you think the project saved you financially to guarantee that the rate of evolution continues.
Membership and Subscriptions
Even though you received the product(s) for free, it never hurts to get a little more in return. By choosing to purchase menbership or subscriptions you offer scheduled income for project and also get products or information in return. The guarantee of additional income helps a project to be able to plan their future finances and decide if they can afford to hire full time employees to allow more focused and structured work to speed up the evolution.
As with the Membership and Subscriptions, by purchasing goods from the project's store you are able to contribute money and receive goods in return. Even though the percentage of profit from the purchases is less than the profits gained from donations, you are able to contribute something greater which is advertising. Consider that through a purchase that only gives a project $5 of profit you are able to direct additional people to the project which will lead to greater contributions from those new users, so your $20 purchase may in fact yield several hundred dollars and new contributors for the project in the long term.
Some projects offer direct paid support, but others may direct you to third party companies for subscription based support. By purchasing a support subscription you are able to help the support group to better document common issues and recommendations for future documentation and implementation. In addition to the information and profit that is gained through support contracts, the support groups may send a portion of your support dues and the output of your issues and recommendations back to the project in consolidated batches.
The Unspoken Contributions
Even though the methods given above result in direct financial support or evolution, you as a user can give back in a way that requires no money and limited time. You as a user can contribute back to the project(s) by filing bug reports and recommendation so they can fix issues and adapt the product to better fit the needs of the current users.
As you have seen from the list there are many ways to give back and each method has its merits, the best recommendation is to give back in a way that results in more long term benefits for the projects, which will help you and the community more in the long term.