You can use Google Spreadsheets to work with your pre-existing worksheets (either native Microsoft Office Excel .xls or comma-separated-value .csv files) by uploading them to Google Spreadsheets.

Nick Carr offers his assessment of Google's evolving Office plan. He posits that Spreadsheets makes Excel more valuable because is provides a simple Web interface to Microsoft's Office spreadsheet ...

Both Google Sheets and Microsoft Excel are powerful spreadsheet software. While in Excel you can work offline, Google Sheets lets you create and manage spreadsheets online. Google Sheets also ...

Google Spreadsheets does support multi-sheet spreadsheets, just like Excel (I was mistaken in my earlier blog post, where I said it didn't).

The online Google Spreadsheets is free, easy to use, and handy for collaboration, but stick with Excel for complex spreadsheets.