2016 update: The easiest way is to use the Google Apps Script API, in particular the SpreadSheet Service. This works for private sheets, unlike the other answers that require the spreadsheet to be published.
This will let you bind JavaScript code to a Google Sheet, and execute it when the sheet is opened, or when a menu item (that you can define) is selected.
Here's a Quickstart/Demo. The code looks like this:
// Let's say you have a sheet of First, Last, email and you want to return the email of the
// row the user has placed the cursor on.
function getActiveEmail() {
var activeSheet = SpreadsheetApp.getActiveSheet();
var activeRow = .getActiveCell().getRow();
var email = activeSheet.getRange(activeRow, 3).getValue();
return email;
}
You can also publish such scripts as web apps.