The main collected editions of Marx and Engels’s work can be found here:

• Marx Engels Gesamtausgabe (MEGA): contents list, PDF.
• Marx Engels Werke (MEW): text, PDF.
• Marx Engels Collected Works (MECW): text, PDF.

Some useful links:

• Marx/Engels Papers – scans made available by the International Institute of Social History, Amsterdam.
• Marx 200 – site dedicated to information on Marx’s 200th birthday in May 2018
• Marx Myths and Legends – useful overview of some frequent misunderstandings
• Marx Bibliography – comprehensive literature overview divided by topic

Below are copies of articles and pamphlets related to Marx that I’ve come across in my research and are not otherwise easily accessible online:

• E. Balfort Bax, ‘Leaders of Modern Thought: Karl Marx’ (1881) – an early biographical and theoretical overview.
• Paul Harro Harring, Historische Fragmente über die Entstehung der Arbeiter-Vereine (1852) – an anti-communist pamphlet that Marx and Engels satirised in their Great Men of the Exile (1852). (Source: IISG, Amsterdam).
• Karl Heinzen, Programm der teutschen Revolutionspartei (1850) (transcribed version) – a draft proposal for a programme for the German republicans in exile in London, which Marx and Engels attacked in Great Men of the Exile (1852). (Source: IISG, Amsterdam).
• Karl Heinzen, Die Rechte und Stellung der Weiber (1852) – book defending the rights of women. (Source: IISG, Amsterdam).
• Karl Heinzen, ‘Herr Karl Marx’ (1859) – a critical review of Marx’s A Contribution to the Critique of Political Economy (1859), which Marx replied to in Capital vol. 1 (1867). (Source: Labadie Collection, University of Michigan).
• Andreas Gottschalk, ‘An Herrn Karl Marx’ (1849) – an attack on Marx during the 1848 Revolutions by a rival communist.
• Marx, Marginal Notes on Machiavelli’s Florentine Histories – Marx probably read and commented on Machiavelli’s book in 1857, writing to Engels that it was a ‘masterpiece’. (Source: RGASPI, Moscow).
• Eduard Müller-Tellering, Vorgeschmack in die künftige deutsche Diktatur von Marx und Engels (1850) – a critique of the supposed authoritarian tendencies in Marx and Engels, and described as the ‘first anti-Semitic pamphlet’ against Marx. (Source: IISG, Amsterdam).
• Arnold Ruge, ‘Der König von Preußen und Socialreform’ (1844) (transcribed version + English translation) – an article in Vorwarts! by a former collaborator of Marx, which questioned the importance of the Silesian Weavers Revolt. Marx used the article as an opportunity to distance himself from Ruge’s position in his ‘Critical Notes on the Article: “The King of Prussia and Social Reform. By a Prussian” (1844).