When open-source is better
Sometimes open-source software isn’t as good as commercial software, it’s better — at least with respect to certain needs. For example, OpenOffice does just about everything that Word will do, but it has a much nicer equation editor, and it’s spreadsheet app (Calc) does a much better job of exporting to CSV. For smaller datasetes, R has a lot of the power of SAS, but it makes it much easier to handle things like finding residuals, performing regression diagnostics and making graphics. And LaTeX — for long, structured documents, there is absolutely no competition.
Of course, all of these apps have their drawbacks — OpenOffice is big and slow, R chokes on big datasets, LaTeX is a bit complicated to use properly. Still, the commercial apps have drawbacks too, and they’re often worse. I think the reason why some open-source apps are so suited for particular tasks is because they have been written by people with a genuine need to perform those tasks. Less features are added, but the really important ones always are, and they’re implemented very nicely.