Aquaporins are tetrameric complexes that facilitate water diffusion through bilayers. They have amazing selectivity as they do not pass other solute or even protons (H3O+ or naked H+ ions). See the amazing link below to molecular dynamics simulations of the process.
Gram negative bacteria (as well as mitochondria in eukaryotic cells) have protein complexes called porins (different from the aquaporin discussed above). The monomer porin forms a trimer in the membrane which forms a pore allowing small solute molecules necessary for bacterial cell growth to pass. The porin proteins share a 16 stranded anti-parallel beta barrel as a common motif. Solute molecules can pass through the pore created by the beta barrel. An example, maltoporin, is shown below:
An Even Bigger Pore: 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! Its 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.