After living here for a year and a half, I'm finally studying italian again.
Last year I studied for about a month at the "Italian in Florence" language school at Centro Internazionale Studenti "Giorgio La Pira". It was a great beginner course, held Monday through Thursday from 9-11am.
This time around I decided to stay a little closer to home, so I tried another school that a friend had recommended, Instituto Michelangelo
on Via Ghibellina. My classes are Monday through Friday from 9-1:00 (otherwise known as 9,00 to 13,00) - the first two hours are grammar and the second two hours conversation. The classes are also very small (usually about 4 to 8 students) and the conversation component I think is especially effective.
There are a million language schools in Florence. I can't comment on most of them, but I would recommend both the schools I've tried so far. Giorgio La Pira is a little less expensive, while Instituto Michelangelo is perhaps a little more intensive - so I guess it depends on what you're looking for.
I've actually had a few neighbors and local shop keepers comment that my italian is improving! So they must be doing something right.