Google released a basic application for Windows that lets you upload files to Google Docs by using drag and drop. The application is more like a sample for Google Docs API, but it's pretty useful if you want to upload a lot of documents. There's also an option to add "Send to Google Docs" to the contextual menu so you can upload files directly from Windows Explorer.
The application can be downloaded from this URL and requires .NET Framework 2.0.
Another way to upload more files at once is to send them to the email address displayed on this page, but this only works with documents and presentations and there are some restrictions regarding the size of the uploaded files: for example, you can send by email only presentations smaller than 500 KB. Google Toolbar for Firefox also has a feature that lets you open files in Google Docs.