After Harry Potter's unfortunate victory at the Triwizard Tournament, he despises the idea of being associated with the event so much that he practically donates his thousand galleon prize money to George and Fred Weasley. This ends up being the best thing possible for the twins because they are finally able to start investing in their dream project.
Weasleys' Wizard Wheezes is born right before the start of The Half-Blood Prince, and it instantly becomes a major hit in Diagon Alley. It made the two men considerably wealthier than the rest of their family and established their name quite firmly in the business world. The shop included a variety of products, the most popular of which were their joke items.
Boxing Telescope
The Boxing Telescope is a simplistic device, but works to prank perfection — it works like a telescope, but if the user presses it even slightly, a tiny spring-based mechanism is released, producing a small boxing glove that whacks them in the face.
The Weasley twins are clearly business-smart, because the blue-black mark generated around the eye is nearly impossible to remove with basic healing spells or potions. In fact, only the "Bruise-removal paste", one of their other items, can do so.
Anti-Gravity & Headless Hats
Hats are apparently a highly popular choice, as they can be worn anywhere without arousing suspicion. One of the enchanted varieties at the shop is the Anti-Gravity subtype, advertised as "Ruin a gentleman's day by making his hat fly away!", suggesting its most common usage.
Headless Hats, on the other hand, make the head of the wearer vanish temporarily, but the magic is of an elite level (considering that even Hermione is awed by its functionality.)
Skiving Snackbox
This is a favorite of Hogwarts pupils, as it allows them to do what they want the most: skip classes. These snackboxes come with a series of different confectionary, each of which has specific patterns and tones depending on their purpose.
Further, they are composed of both the problem as well as the solution, to be consumed after the student has successfully convinced the teacher that they are suffering from whatever invented medical condition.
Fanged Flyer
The Fanged Frisbee works the exact same way as the ordinary variant, except that it's kind of sentient and extremely aggressive. It seems that they burst out an array of terrifying canines, accompanied by predatorial growling, presumably to scare whoever it's being thrown at.
This item is one of the few banned at Hogwarts, at least according to Argus Filch, the caretaker. The Weasley version is known as the Fanged Flyer, which is identical in all respects, but also comes with a "handler's glove" for obvious reasons.
Dungbomb
The Dungbomb is also a well-known prank item, but is taken to new levels by Fred and George Weasley. In fact, it is the main cover used by the twins while filching the Marauder's Map from Filch's chambers.
Further, the students rebel against the dictatorship established by the Ministry under Umbridge, by simply throwing a large number of these items whenever they could. The original cost a Sickle and 21 Knuts, so it can be assumed that the price hasn't changed much at Weasleys' Wizard Wheezes, either.
Canary Creams
This delicious biscuit (consisting of a filling of standard creamy pudding) costs 7 Sickles, a vastly inflated price compared to its unenchanted counterpart. This is because it is constructed with ridiculously advanced transfiguration magic, as consuming it will convert the person to a gigantic canary bird.
Neville accidentally falls for this trick in the Gryffindor Common Room — during the victory party after Harry's success in obtaining the dragon's golden egg in the fourth book. Luckily the charm disintegrates in a few seconds.
Reusable Hangman
This toy is based on the hangman game that is popular amongst Muggles, except it is an actual physical object, as opposed to the hand-drawn version. Advertised as "Spell it or he'll swing!", the Reusable Hangman includes a tiny humanoid figure made of wood that slowly inches towards an equally tiny gallows every time the user makes a spelling error.
It is in incredible demand for its unique style, as evidenced by the sheer number of bulk purchases made every month at the Weasley twins' store.
Trick Wands
A rather special item at Weasleys' Wizard Wheezes is the trick wand, which looks like the original so much that they end up fooling most people. However, if anyone employs them to produce magic, the wands transform into a host of hilariously inane things, the most common of which are rubber chickens.
In fact, Harry and Ron's mock duel with these toys in The Goblet of Fire turn into a rubber haddock and a tin parrot, respectively. There is a costlier variant of the trick wand, except this one attacks the caster.
Portable Swamp
The Portable Swamp, as the name suggests, is a small object that manifests itself as a localized bog wherever they are placed. First introduced in The Order of the Phoenix, the Weasley twins "install" it in the hallway leading to the High Inquisitor's chambers, which Umbridge totally fails to clear away.
It's not that the magic used is so powerful, as McGonagall, Snape, or Flitwick could have vanished the Portable Swamp in a second, but it's just a lot more fun seeing the latest antagonist go through all that hassle.
U-No-Poo
This item is very basic, and just works as a charmed stool hardener. Appearing in tablet form, it can be administered to the prank victim by blending it into their meals.
While U-No-Poo is not exactly impressive by itself, what makes it the best Weasleys' Wizard Wheeze is its comically dangerous slogan "Why are you worrying about You-Know-Who? You should be worrying about U-No-Poo!", which Molly thinks is far too risky given the present climate — she is terrified that her twins are going to get "murdered in their beds" for it.