The IRIS database is a digital collection of instruments, materials, stimuli, data coding and analysis tools that have been used for research into second languages and multilingualism. Many of these materials can be employed in studies with first language learners and monolinguals as well. Materials are freely accessible, searchable, and downloadable. https://www.iris-database.org
The International Picture Database is the result of a large international study to provide norms for timed-picture-naming in different languages and cultures (American English, German, Mexican Spanish, Italian, Bulgarian, Hungarian, and Mandarin Chinese (Taiwan variety). Pictures can be searched by lexical parameters, percent name agreement, response times, and visual complexity. http://crl.ucsd.edu/experiments/ipnp/
The Multilingual Picture (MultiPic) databank results from an international collaborative project intended to provide the scientific community with a set of publicly available 750 drawings from common concrete concepts created by the same author, standardized for name agreement and visual complexity in several European languages: https://www.bcbl.eu/databases/multipic/
The clipart collection of Florida’s Educational Technology Clearinghouse (ETC) offers thousands of illustrations arranged in galleries (alphabets, animals, music, plants, etc.). For every item, you have a choice of image size and format. Some of the materials are free for educational use. http://etc.usf.edu/clipart/
The ARC database provides English-style non-word stimuli:
Rastle, K., Harrington, J., & Coltheart, M. (2002). 358,534 nonwords: The ARC Nonword Database. The Quarterly Journal Of Experimental Psychology. A, Human Experimental Psychology, 55(4), 1339-1362. http://www.cogsci.mq.edu.au/~nwdb/nwdb.html
The English Lexicon Project offers a large set of lexical characteristics and behavioral data from visual lexical decision and naming studies of 40,481 words and 40,481 nonwords. It collects normative data for speeded naming and lexical decision for over 40,000 words across 1200 subjects at 6 different universities.
Balota, D. A., Yap, M. J., Cortese, M. J., Hutchison, K. A., Kessler, B., Loftis, B., et al. (2007). The English Lexicon Project. Behavior Research Methods, 39(3), 445-459. http://elexicon.wustl.edu/
If you want to check whether a potential non-word does not already have a meaning in current English usage, you might want to check the Urban Dictionary: http://www.urbandictionary.com
The pseudoword generator Wuggy makes pseudowords for psycholinguistic experiments in Basque, Dutch, English, French, German, Serbian (Cyrillic and Latin), Spanish, and Vietnamese.
Keuleers, E., & Brysbaert, M. (2010). Wuggy: A multilingual pseudoword generator. Behavior Research Methods, 42(3), 627-633. http://crr.ugent.be/programs-data/wuggy
WordGen uses the CELEX and Lexique lexical databases for word selection and nonword generation in Dutch, English, German, and French to generate items specifying any combination of seven linguistic constraints: number of letters, neighborhood size, frequency, summated position-nonspecific bigram frequency, minimum position-nonspecific bigram frequency, position-specific frequency of the initial and final bigram, and orthographic relatedness.
Duyck, W., Desmet, T., Verbeke, L. P. C., & Brysbaert, M. (2004). WordGen: A tool for word selection and nonword generation in Dutch, English, German, and French. Behavior Research Methods, 36(3), 488-499. http://users.ugent.be/~wduyck/Wouter_Duyck/wordgen.html