This includes ACOL, but there is also a modern variant.
In this system, a major suit opening is a 5-card suit, or a 4-card suit in a 12-14 balanced hand.
This way of bidding is standard in the USA, France, Germany and many more countries. There are several variations.
The main are:
A system opening best minor: Standard 5-card majors If you want 1 as natural: Preparatory Club. This is my preferred method, and I will present the system in more detail. A novel way of playing this is the Dull Club system.
Especially good for beginners is the Romex-style Big NT system, which has no conventions, not even Stayman!
There are two main streams here. One way of playing this is to include all strong hands in an otherwise semi-natural 1-bid, as is done in Polish Club, and, if you like weak NT, Unassuming Club.
One can also play 2 as strong and put all the hands in 1. An example of this is Dutch Doubleton
In this category is the well-known system Precision. Precision-type systems lend themselves for relaying. To learn about relaying check out Symmetric Relay Precision. A very nice write-up of the complete relay system by Andrei Sharko is available here.
The "Strong Club" category also includes systems where 1 is either a strong hand, or a weak balanced hand.
This has the goal of protecting the strong club from weak overcalls.
An example is Carrot Club. If you want to go even further, you can play a more agressive variant where 1 includes a mini NT and 1 becomes even more vague. The Viking Club is such a system.
The ultimate agressive version of Precision is OltBrink. If you thought strong club systems are meek, better check this one out!
The cornerstone of the system are the forcing and unlimited 1-level opening bids (1 through 1!). As this is a meek version of EHAA, I will call it "Every 2nd hand an adventure" or E2HAA.A description can be found here.
This type of systems is for people who hate passing all the time. Neither 1 and 1 show any distributional information. 1 is very strong, 1 shows an opening. The other bids are weak.
Magic Diamond is a system in this category. I enjoy playing this system, and the page includes the version I play.
Although banned in most tournaments, these kind of systems are very interesting. The most famous example is Polish Pass, the most successful example of this is Suspensor. In this system, pass is used for all 13+ hands. The same is true for the relay-system Eclectic Symmetric Pass. One can also make strong pass systems by changing a right-way-up system a little bit. For example one can change a 5-card major system into the Tuebingen Pass system.
A good way to make a strong pass is by rearranging the bids in a strong diamond system. If one does this to Magic Diamond, one gets a system that may be called Magic Pass. A variant of precision is the Precision Pass.