Flexible grid system is important for building responsive web. Right now there are a lot of grid systems. Twitter Bootstrap is probably the most popular right now. It is also the most watched project at Github. However it is not just grid system, it is a UI framework, it includes tools for UI components.
If you are only looking for a grid system, Semantic Grid is a good choice for fluid grid. It has 12 columns. It doesn’t use named class (like grid_1, etc), so your HTML wont be poluted with presentation class. It utilizes LESS mixin to achieve the same effect.
Golden Grid System is also a good fluid system, though it is more of a guideline. It uses 16 columns for design.