As annk says, just redownload and install the latest combo update to OS X. Usually puts back whatever should be there.
(My Grandad deleted Mail.app once on his iBook!...don't ask how!...a quick combo update back to what was 10.4.6 I think back then, sorted him out).
I had iSync trouble a few weeks back. Reinstallation of 10.4.8 sorted me out.