I report bugs whenever I encounter them, and am ready to make screenshots, answer follow-up questions and dig my way through less-than-perfect bug reporting systems. I contribute ideas and suggestions when I have the feeling they will be listened to.
I am currently not in the situation to make larger donations, but plan to make some as soon as I can, most of all to the creators of Subversion, TortoiseSVN, and some other products I work with every day.
I have worked for companies and have found that while there is often the will to contribute to the community - at least the tech people almost always realize the huge amounts of money saved by using OSS - there are big procedural, legal and tax obstacles. If you have a purchase department, it can be difficult to justify a donation when the actual product is free of charge. Also, it is difficult to declare donations as business expenses, rendering any kind of contribution impossible in many companies. I have thought about this often and think that the Open Source world could profit hugely from introducing solutions to this without abandoning its general principles, e.g. by setting up national non-profit foundations that receive and channel contributions to various OSS projects.
Hassan Syed mentions another important reason why there is not more support coming from businesses. I think businesses are often very ready to release code that has been written inhouse, as there is mostly a huge benefit to the business itself, because there is a chance the product will be maintained, enhanced, bug fixed and updated for free by a larger community. But to properly release a project, it's never enough to just dump it somewhere on Google Code. It has to be security checked at the least, to prevent the own inhouse solution from becoming suddenly vulnerable to attacks by losing security by obscurity. And you have to put considerable effort in setting up and promoting a project if you want it to actually catch on, and not just rot somewhere. Businesses often don't have, or can't get authorized, the necessary resources to do all this. Open Source engagement of any kind is often very difficult to justify upstairs. This could be a market for a "Summer of code" like initiative: Students interning at companies, taking their in-house solutions Open Source, under close supervision by the company's IT. I bet there are a lot of gems that could be unearthed that way.