Google launched a new API, this time for Google Docs & Spreadsheets. The API allows you to upload files, obtain a list of all the documents and perform searches programmatically. You can authenticate securely, without passing your credentials to the applications.
Google Documents List Data API is too limited to use it for creating a Microsoft Office plug-in that synchronizes a list of documents, but it should be enough for adding an option to open a document directly in Google Docs, without downloading the file. It's also possible to create a new file manager for Google Docs or to build your own GDrive by adding the photos from Picasa Web Albums and the posts from Blogger in a single interface. More complex options can be found in the spreadsheets API, that lets you access and edit a spreadsheet at the cell level.
As observed by Tony Ruscoe, Google changed the terminology for files: you'll work with word processor documents (instead of just "documents") and spreadsheets. The generic name for files that can be edited by Google Docs is documents: this could be another step that prepares the introduction of Google's presentation app and another name change, this time from Google Docs & Spreadsheets to Google Documents (or Google Docs).