I'm a blogging machine today... Google Docs are awesome! We actually currently use the spreadsheet functionality and we used the docs when planning our wedding planning.
Spreadsheets are used for clients listing, developing of their families, and account numbers.
For personal training- contracts, training plans are produced for word documents. Spreadsheets are a great way to track work-out progress.
For wedding planning we used them to sign contracts, and guest lists.
Also, it is great to open documents on the web without having to download the documents if you are briefly reading a document.