Here are a couple of lists of FLOSS (Free/Libre Open-Source Software) that are decent (in most cases) replacements for other proprietary software that you may use. The review at Simple Dollar lists 30 essential applications that you really should be using; OS-Alt has a more extensive list that you can search for software that serves a precise need. Bear in mind that although it may not be as fully featured as something you pay for from a big corporation, you’re getting it for free, it’s likely to be less invasive and more secure than more heavily integrated software, and that if you want to contribute an improvement or a bug, the whole community of users will benefit.