Which open formats can I use?

The easiest way to read, save and exchange text is to use plain text.
Plain Text

If you need to exchange documents with images and formatted text, the best solution is to use HTML format.
Hyper Text Markup Language (HTML)

Math documents edition and formatting uses a dedicated format: TeX.
TeX, LaTeX ad Device Independent format (DVI)

If you need to collaborate with a team to work on the same document, but you cannot handle HTML files or need features not supported by HTML, you should use the Open Document Format - the document standard for office applications - or a semi-open alternative like the Rich Text Format:
Open Document Format
Rich Text Format (RTF)

If you need to present or print high quality documents, PostScript (PS) and Portable Document Format (PDF), are solutions of choice.
PostScript (PS)
Portable Document Format (PDF)

Main open formats available to store images are following:
Joint Photographic Expert Group (JPEG)
Portable Network Graphics (PNG)
Scalable Vector Graphics (SVG)

Available open audio formats include:
Ogg Vorbis (OGG)
Musepack (MPC)
Free Lossless Audio Codec (FLAC)
Speex