People know Adobe because of the Create Suite, PhotoShop, Acrobat, InDesign,... but it all started with a small company that developed the PostScript language and a font specification named Type 1. The products we all know and love didn't exist yet, in its early years, Adobe made money selling fonts.
That was a really long time ago. Adobe was founded in 1982, I was 12 years old at that time (I bought my first computer at that age, a TI-99/4A). Thirty years later, Adobe released its first open source font: Source Sans Pro.
You can download the font from SourceForge. There are two sets of 12 fonts:
As you can see, there are the four usual font (Regular, Bold, Italic, BoldItalic), extended with two Black fonts, two SemiBold fonts, two Light fonts, and two ExtraLight fonts. I've immediately tried using them in an iText application, and the result was really nice. You can download the code to create this PDF here.
Need more fonts, check out my useful links about fonts list in Appendix B of iText in Action — Second Edition.