Iotop displays disk I/O usage of individual process, in a way similar to top. What I really loved though is that it allows one to display the accumulated amount of I/O on any of the DISK READ, DISK WRITE, SWAPIN, and IO (overall). This is through a nifty interface:
The presentation is rather busy, and it doesn't have the myriad options that top has (e.g. I can't chose to hide any of the columns I'm uninterested in), but it's more than good enough for it's specific purpose.