In the Modern period, Kantianism gave rise to the German Idealists, each of whom had their own interpretations of Kant's ideas. Johann Fichte, for example, rejected Kant's separation of "things in themselves" and things "as they appear to us" (which he saw as an invitation to Skepticism), although he did accept that consciousness of the self depends on the existence of something that is not part of the self (his famous "I / not-I" distinction). Fichte's later Political Philosophy also contributed to the rise of German Nationalism. Friedrich Schelling developed a unique form of Idealism known as Aesthetic Idealism (in which he argued that only art was able to harmonize and sublimate the contradictions between subjectivity and objectivity, freedom and necessity, etc), and also tried to establish a connection or synthesis between his conceptions of nature and spirit.
Arthur Schopenhauer is also usually considered part of the German Idealism and Romanticism movements, although his philosophy was very singular. He was a thorough-going pessimist who believed that the "will-to-life" (the drive to survive and to reproduce) was the underlying driving force of the world, and that the pursuit of happiness, love and intellectual satisfaction was very much secondary and essentially futile. He saw art (and other artistic, moral and ascetic forms of awareness) as the only way to overcome the fundamentally frustration-filled and painful human condition.
The greatest and most influential of the German Idealists, though, was Georg Hegel. Although his works have a reputation for abstractness and difficulty, Hegel is often considered the summit of early 19th Century German thought, and his influence was profound. He extended Aristotle's process of dialectic (resolving a thesis and its opposing antithesis into a synthesis) to apply to the real world - including the whole of history - in an on-going process of conflict resolution towards what he called the Absolute Idea. However, he stressed that what is really changing in this process is the underlying "Geist" (mind, spirit, soul), and he saw each person's individual consciousness as being part of an Absolute Mind (sometimes referred to as Absolute Idealism).
Karl Marx was strongly influenced by Hegel's dialectical method and his analysis of history. His Marxist theory (including the concepts of historical materialism, class struggle, the labor theory of value, the bourgeoisie, etc), which he developed with his friend Friedrich Engels as a reaction against the rampant Capitalism of 19th Century Europe, provided the intellectual base for later radical and revolutionary Socialism and Communism.
A very different kind of philosophy grew up in 19th Century England, out of the British Empiricist tradition of the previous century. The Utilitarianism movement was founded by the radical social reformer Jeremy Bentham and popularized by his even more radical protegé John Stuart Mill. The doctrine of Utilitarianism is a type of Consequentialism (an approach to Ethics that stresses an action's outcome or consequence), which holds that the right action is that which would cause "the greatest happiness of the greatest number". Mill refined the theory to stress the quality not just the quantity of happiness, and intellectual and moral pleasures over more physical forms. He counseled that coercion in society is only justifiable either to defend ourselves, or to defend others from harm (the "harm principle").
19th Century America developed its own philosophical traditions. Ralph Waldo Emerson established the Transcendentalism movement in the middle of the century, rooted in the transcendental philosophy of Kant, German Idealism and Romanticism, and a desire to ground religion in the inner spiritual or mental essence of humanity, rather than in sensuous experience. Emerson's student Henry David Thoreau further developed these ideas, stressing intuition, self-examination, Individualism and the exploration of the beauty of nature. Thoreau's advocacy of civil disobedience influenced generations of social reformers.
The other main American movement of the late 19th Century was Pragmatism, which was initiated by C. S. Peirce and developed and popularized by William James and John Dewey. The theory of Pragmatism is based on Peirce's pragmatic maxim, that the meaning of any concept is really just the same as its operational or practical consequences (essentially, that something is true only insofar as it works in practice). Peirce also introduced the idea of Fallibilism (that all truths and "facts" are necessarily provisional, that they can never be certain but only probable).
James, in addition to his psychological work, extended Pragmatism, both as a method for analyzing philosophic problems but also as a theory of truth, as well as developing his own versions of Fideism (that beliefs are arrived at by an individual process that lies beyond reason and evidence) and Voluntarism (that the will is superior to the intellect and to emotion) among others. Dewey's interpretation of Pragmatism is better known as Instrumentalism, the methodological view that concepts and theories are merely useful instruments, best measured by how effective they are in explaining and predicting phenomena, and not by whether they are true or false (which he claimed was impossible). Dewey's contribution to Philosophy of Education and to modern progressive education (particularly what he called "learning-by-doing") was also significant.
But European philosophy was not limited to the German Idealists. The French sociologist and philosopher Auguste Comte founded the influential Positivism movement around the belief that the only authentic knowledge was scientific knowledge, based on actual sense experience and strict application of the scientific method. Comte saw this as the final phase in the evolution of humanity, and even constructed a non-theistic, pseudo-mystical "positive religion" around the idea.
The Dane Søren Kierkegaard pursued his own lonely trail of thought. He too was a kind of Fideist and an extremely religious man (despite his attacks on the Danish state church). But his analysis of the way in which human freedom tends to lead to "angst" (dread), the call of the infinite, and eventually to despair, was highly influential on later Existentialists like Heidegger and Sartre.
The German Nietzsche was another atypical, original and controversial philosopher, also considered an important forerunner of Existentialism. He challenged the foundations of Christianity and traditional morality (famously asserting that "God is dead"), leading to charges of Atheism, Moral Skepticism, Relativism and Nihilism. He developed original notions of the "will to power" as mankind's main motivating principle, of the "Übermensch" ("superman") as the goal of humanity, and of "eternal return" as a means of evaluating one's life, all of which have all generated much debate and argument among scholars.