What is an "Organiser"? An organiser should be something that makes your life simpler, and allows you to access information in a structured way. It may be simple list of tasks to do, birthdays to remember, appointments to attend or assignments to hand in. aids planning and memory. However, the quality of the "output" will always be dependent on the input!
Most people have some form of "organiser", but the problem is that the good intentions rarely last. Many start of with simple "stickies" (little yellow squares), until the screen is full. Then they try another, which maybe sends email to their smart phone, only to find that they still forgot the task as they were unable to do it immediately. And many return to the old fashioned notebook.
But why is this? It is because everybody has their own needs and preferences, and probably the only way they will every succeed (if ever!) is if they a) find one that is perfectly in line with the way they work, or b) they design their own. And of course now they can be online as well as offline.
So, how do you choose? The problem is that until you have tried it, you do not know which is the best. There is not even much point in taking somebody else's recommendation as what works for them is unlikely to work for you. And sometimes they are so "all powerful" that it is difficult to know where to start.
There is no simple answer, but there is choice. And while choice is not always a good thing, here is a website of 100 online Organisers and Task Managers, complete with screen shots and brief descriptions.
List of Online Organisers and Task Managers