osalt.com is a site that lists Open Source alternatives to commercial applications. It is broken down into the following categories:
- graphic applications
- internet & networking
- multimedia & audio
- security & privacy
- system utilities
- web development
While most commercial products have an free alternative, there are sometimes features that the commercial product offers the the free one doesn't. For instance, you can generate PDF's with PDFCreator, but if you want to edit them (or add form fields), Adobe Acrobat is the only real option.
Sometimes the Open Source counterpart can be more unstable and lack technical support (there will be exceptions to that though - generally when the release version is greater than 1.0). With each release though, stability normally gets better.