Everybody have different budgets they can spend on their computers and applications. Invariably most have a gap in the basic software you can use to produce roleplaying stuff. So I went around and looked at the best of the open source alternative to commercial applications.
For an Office suite with a Word Processor, Spreadsheet, Database, and Presentation software there is OpenOffice
For a Word Processor there is Abiword which feels like an older version of Microsoft Word. Note I prefer Abiword over the OpenOffice word processor which feels too busy for me.
For Spreadsheets there is GNumeric which does a fair job of importing Microsoft Excel files.
For a simple paint program I recommend Paint.NET. There Gimp but the learning curve is very steep.
For a vector drawing program (like CorelDraw or Adobe Illustrator) there is Inkscape. Great for drawing maps especially in conjunction with Paint.NET.
For desktop publishing software to professionally layout a manuscript there is Scribus.
Finally for all the gearheads out there I recommend SMath in conjunction with a spreadsheet like Gnumeric. Smath replicates writing formulas on a piece of paper with the added benefits of being able to calculate those formula. It is pattern after a piece of software called MathCAD and the two are unique in the world of Math software. It is MUCH friendlier for the math savvy gamer than FreeMat or the other math packages. It also invaluable for converting between different units which is often a pitfall for messing around with formulas.
So if you are missing something look at the list and pick it up.
Graeme Morris's GURPS bibliography
3 minutes ago