How do you spell the year 2010?

The year has been spoken "two thousand ten" or "two thousand and ten."

However, the traditional form for YEARS is the hundredsname, so the standard form (historically) will likely become "twenty ten" (and twenty-oh-nine for 2009). The use of the "thousands" form is a carryover from the year two thousand (2000) which like the years 1000 AD and 5000 BC would use the cardinal number.

It is easier to say "the year ten sixty-six" for 1066, rather than "one thousand sixty-six" and 1865 would practically never use the numeral name.