I'm going to start with the obvious one, as its the first one most people think of. Many addon authors have set up ways for appreciative players to donate money to their cause. One way that has been suggested to me is to donate a small amount to a different developer each month. For example, you could consider your World of Warcraft cost to be $20 a month, rather than $15, with five dollars going to whichever author you choose each month. Nothing packs as much punch as money, but not everyone can afford this. The fact is, most developers aren't creating addons for profit, it usually comes down to a labor of love, but I don't think you'll find many that would turn money away, as it can help defer the costs of software purchases, hardware upgrades or just pay for their time. Either way you look at it, there is a real financial cost associated with writing addons.
For those of you not able or willing to part with more of your hard-earned money, there are other ways to support developers. One way, which can often be more valuable than donations, is involvement. As authors release new versions or new addons, there is a need for people to test them and provide feedback. This feedback is what enables developers to put the finishing touches on a release version, often bearing user-suggested changes and features. Wish that threat meter would double as a damage meter? Suggest it to the dev, maybe they aren't sure if there is a audience for that. (Alternatively, it might just be too much of a hassle to code it,) Maybe you've got a knack for noticing little bugs or moments when an addon doesn't seem to perform well. The more people who are actively testing an addon, the better chance it will be released without bugs and with a full compliment of features.
One new method of seeing this in action is WoWInterface's new SVN system. (SVN stands for SubVersioN) For you developers, this is a great tool you may already be familiar with, but its also designed to help users get their hands on, and test, new versions of addons. It also allows users to test different branches of an addon's development, all of which serve to help authors release the best possible product. (With yet another /salute to Cairenn over at WoWInterface for being a rockstar in the addon community!)
So, you could get involved in the actual testing of an addon, you could simply provide feedback via features requests or bug reporting, or you could simply post a "Thank-you" to the developer of a specific addon. Honestly, a craftsman likes to hear that his (or her) work is appreciated.
Regardless of how you go about it, whether building a support network for developers (ala Cairenn), donate money, say "Thank You" or help authors test their projects, there are ways to give back to the community that provides us with so many enhancements to our gameplay. (As for myself, I do a little of each.)
How else might you help addon developers help you? I'd love to hear some other ideas, as I think it is an aspect of our gaming we take for granted.