Though there may be several brands that do a decent job, the one flocculant that seems to consistently work well is OMNI Liquid Floc Plus and the one clarifier that seems to consistently work well is BioGuard Polysheen Blue. This is just what I've seen from reports of people using various products and is not an endorsement (anyone that knows me knows I'm not a fan of BioGuard given Chemtura's policies on CYA). PolyQuat 60 is also a clarifier, but isn't as good for that function as specialized clarifiers.

Of course, neither a flocculant nor a clarifier is necessary if one maintains a shock level of chlorine, brushes the pool, vacuums out larger debris, and runs a good filter 24/7. There are some occasions where this won't work well, either because the filter isn't very good (sand filters don't do as well as cartridge or DE, but you can add some DE to a sand filter to improve it) or because the cause of cloudiness is very fine particles that don't get oxidized by chlorine nor trapped in the filter.