Some of them even come free with other pieces of software. Lots and lots of magazines are very useful for inspiring RPG plots, especially if you're into technology plots. If your players don't keep up with all the latest developments then magazines make it easy to stay several steps ahead of them. If you're like me and you don't have the time and money to keep up 20 magazine subscriptions, just get a year here or there of one or another. As with history books, you often don't have to worry about perfect accuracy when writing fiction, so even the sensationalist magazines can be useful.
Newspapers also make handy resource materials, as well as on-line news services.
A web browser and a decent working knowledge of how to use a search engine are invaluable. Other than that, it all depends on your subject.