Old style sheets
They work online only and limited to about 400,000 cells per spreadsheet.
Old style URL
https://docs.google.com/spreadsheet/ccc?key={some long key here}&usp=drive_web#gid=0
New style sheets
Released about mid Dec 2013
Works offline and (if I remember) up to 2,000,000 cells per spreadsheet.
https://docs.google.com/spreadsheets/d/{some long key here}/edit#gid=0
Spreadsheet KEY
I get the key using Google-apps-script, as described here: Get the spreadsheet key that is in the URL. Not ss.getId()