Phew, may I present for your delectation the aptly titled Farms' Really Big Theme Pack :)
All in all there are 65 themes there ranging from the classic to the ultra modern.
They're all widgetised (i.e. have widgets), all have nice pithy write ups for the Presentation menu, all have been tested and many of them also have extra functions like changeable headers.
They've incorporated in full Nevans' fixes from the last pack and are all running on several live sites (with no shortage of guinea pigs) so if there are any security weaknesses or bugs I should know about them first!
Anyway, enjoy: http://wpmudevorg.wordpress.com/project/Farms-Really-Big-Theme-Pack
And let me know any problems, ideas or important themes I've missed (and before anyone says anything - if someone can post a secure WPMU version of K2 sans bugs and with features I'll include it in the next version).
BTW = The next pack planned is an ultra modern, slim 'un full of themes with changeable headers built in... hopefully sooner than 3 months away :)