Exactly one choice: OpenDocument
...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!