The Nuclear Pore Complex
Channels have pores which can be gated open and allow selective flow of ions. Porins have larger entrances which allow larger molecules to pass the bilayer. The biggest pore structure known is the nuclear pore complex, which has a combined molecular mass around 125,000,000! It's job is to shuttle small molecules through passive diffusion down a concentration gradient through the pore. In addition it moves large molecules and molecular structures (proteins, RNA, and perhaps ribosomes) across the nuclear membrane in a process which requires energy. The proteins that comprise this complex are called nucleoporins, of which there appears to be around 30 in yeast. Large proteins that pass through the pore must first be bound to a cargo receptor, which can move the "cargo" across the pore with concomitant GTP hydrolysis.
Figure: Movement of Molecules and Particles Through Membranes