Over the past six months it seems that there has been a large push for responsive design so I felt it necessary to layout a few different options for your next WordPress theme. Below is a list of recommended frameworks and grid systems to get started with.
12-column grid with fixed and fluid width layouts
960px grid system
1140px fluid width system
12 column fluid system that starts at 1280px
960px grid with fluid or fixed layouts
12 column fluid system
The above list is by no means comprehensive although I have personally used many of these options and would recommend them for your next WordPress theme. Feel free to take a look at these options and let me know what you prefer to use for your themes.