Due to the weekly postings asking about the purchase of Uffizi (and Academia) tickets online, I'm posting this information here and under the "Florence FAQs".
There is only one OFFICIAL uffizi ticket buying website. It is:
This is run in partnership with Firenze Musei (http://www.firenzemusei.it), with whom you can also book by telephone (+39 (0)55 294883.
Any other website charges a booking fee and is not the official site.