Exactly one choice: OpenDocument

Tim Bray:

...if you occasionally create documents or spreadsheets or presentations, and if you think that you’d like to own them, independent of your Office software vendor, well, you have exactly one choice: OpenDocument...

The OpenDocument v1.0 format was approved a few days ago by OASIS, and its schema is published in RelaxNG.

Tim also says that they're committed to making OpenDocument an ISO standard, which should make a significant difference for its acceptance in Europe.

Looks like something to stay tuned to!