C-x C-b (M-x list-buffers) displays a list of all the buffers. Modified ones will be shown with a * next to them. You can mark a buffer for saving by pressing s. When you're done, press x to save all the buffers you marked.
Unfortunately, as far as I know, there's no way to show only the unsaved buffers or to sort them so they're all at the top.
(I actually prefer M-x ibuffer to M-x list-buffers, but ibuffer provides a similar feature.)