Flat pedals are great for lots of reasons, but I won't get into the virtues or pitfalls of platforms versus toe clips versus clipless systems (though I am a big fan of plain old platform pedals.) I will try to give information pertaining to the different styles and a few examples rather than an exhaustive list of specific brands and prices. There are lots and lots of different models which are mostly variations on two themes: rat trap quill pedals and one-piece platform pedals. None are perfect, but many are cheaper and easier to use/maintain than clipless pedals.
Classic rat trap pedals have been around for a long time - one or two metal sides secured to a central body and quill (there were lots of poor quality pedals spec'd on bikes back in the day, but modern ones are generally of decent quality.) Variations use the same style of construction to much the same effect. Some people balk at using pedals without sealed bearings, but bearing maintenance on pedals is probably the easiest to tackle and shouldn't be an issue for most people. The VO Road pedals above use sealed bearings, while the MKS Sylvans use loose bearings. I would say they are both of reasonably high quality. They accept toe clips and reflectors and other forms of toe retention and are super easy to use (platforms on both sides so you can't miss.) A few brands make pedals that fold up so as to not catch on anything (MKS FD-7s...important if you have to carry your bike indoors frequently.)
Two drawbacks: these pedals are generally not as grippy as some others, noticeably in the rain/snow. Second is that the construction can be felt through thin shoes. In Chuck Taylors with thin socks I found the dip in the MKS Sylvans to be a bit uncomfortable. Generally folks don't mind it, though.
Track pedals are narrower and only have a platform on one side as they're intended to be used solely with toe clips and straps. These are great if you love toe clips, but there is no back side to the pedal so they're somewhat limited in flexibility. On the plus side, they're lightweight and narrow so that you can clear obstacles (cars, curbs, doorways, people's ankles...)
There are also many machined and forged one-piece pedals based on the above designs (plastic versions I've seen are not worth consideration.) Velo Orange Touring pedals are surprisingly light weight and use sealed bearings (their narrower Urban pedal would be comparable in width to a track pedal but still has two useable sides.)
BMX style platform pedals are super popular and work for lots of people. The cheaper ones are probably the most affordable/functional pedals out there. These pedals range from fat and chunky plastic to graceful CNC aluminum (and there are lots of garish colors out there for those into color-coordination.) Some of these pedals have provisions for reflectors and most allow for strap-type foot retention (but not toe clips.) The more expensive pedals will have sealed bearings, but you generally get what you pay for. The big benefit of this style of pedal is that they have a nice flat platform with lots of grip. Metal platforms frequently have pins set into the tops to add grip which, if you've ever ridden these pedals in shorts, leads to frequent bloody shins (so avoid expensive pants or delicate leg skin.)
Cruisers and utility bikes will sometimes have plastic or composite pedals with rubberized tops that don't tear your shins into ribbons. I have a set of VP 001 Thin Gripsters and they're great for commuting and hauling around town. They do have sharp little nubbins, but I can deal with it as they're really shine in the rain and snow. They're noticeably thinner than other platforms (hence the name) but they're wide and long so I never misstep.
The big drawback to these pedals is that they're heavy and wide (plus the bloody shins, but I covered that.) The weight is a compromise in any case, but the width can sometimes be a real problem i.e. a low bottom bracket with long cranks will dig your pedal into the pavement while cornering. Similarly, riding a bike with lots of toe overlap on the front wheel will lead to endless suffering in tight places. These can be avoided with careful maneuvering but will surprise the unawares.