Since January 1, 2004—the beginning of the Saturn phase of the mission—Cassini has radioed almost 139 Gigabytes of science data to Earth. Which, if you think about it, doesn't really sound like that much; between my computer's hard drive and my spare networked hard drive, I could easily store all of that myself. Of course, the data that Cassini has returned to Earth has been run through all sorts of compression algorithms -- different for each instrument -- to pack as much information as possible into every byte that must be laboriously transmitted across a billion kilometers of empty space and received at the world's largest radio dishes before being sent to scientists. Once on the ground, the data is unpacked into formats that are easier to use but occupy much more space. As one example, Cassini's camera images usually contain a lot of blank black space; compression algorithms can very efficiently squeeze that blank blackness into just a few bytes, saving the precious data volume for interesting things like storms on Saturn and waves in the rings.
This tidbit of information comes from the latest weekly report of Cassini Significant Events. The report also included a table comparing the data volumes transmitted by all of Cassini's science instruments. (Note that this doesn't count the other housekeeping data that the spacecraft also returns.)
Here's more information on Cassini's instruments and the difference between the optical remote sensing and in situ fields and particles investigations.
By the way, Cassini is presently zeroing in on a Sunday targeted flyby of Titan, which will include RADAR imaging of Xanadu. The RADAR coverage will be an equatorial swath almost perfectly complementary to the one they got on flyby T8 back in October. I can't wait to see that RADAR data -- I'll post it as soon as I see it, I'm hoping early next week.