Do you know any iPad compatible spreadsheet (preferably free) that allow to sort data (like in Excel). iSpreadsheet and Excel Notepad does not have this feature.

Thank you!

Almost all the good ones do, or so I hear. To the best of my knowledge there are no 'good' and 'free' options. Well, maybe CloudOn and OnLive, but those only work if and while you have a good internet connection. If you can live with that, give them a try.

Otherwise, the better apps are:

iWorks (Pages, Numbers, Keynote) (my choice)
Documents to Go
QuickOffice
Office HD
Polaris Office.