If you want to do a quick look-up of any soil dwelling nematode or a plant parasitic nematode genus or species and learn the basics of their lifestyle, Nemaplex is an awesome resource. It is created and curated by Dr. Howard Ferris from University of California, Davis.
Caenorhadbitis elegans or C. elegans is a bacterial feeding, free living nematode. This was in fact the first ever animal whose genome was completely sequenced more than 2 decades ago. This is a simple animal and easy to work with, and has been used as a model organism. A lot is known about this nematode and its very close cousins. If you want to look-up something or anything about C. elegans or its cousins, wormbase is a good place to go to.
If you are interested in a certain nematode or any agricultural pest, and if you would like to do a quick look-up of its distribution across the world, the EPPO database is the place to be.
If you are curious about nematodes that cause diseases in animals and want to learn more about them, Nematode.net is the place to be. This database also curates agricultural nematodes. Furthermore, if you want to analyse genomic and gene expression data, this site has those perks and a good GUI for that.
Last but not the least, here is an entire database dedicated to my favourite nematode, soybean cyst nematode (SCN), SCNbase. This database holds all kinds of genetic and genomic information on SCN and also offers a BLAST feature to search for favourite genes. An awesome effort by the Baum lab at Iowa State University. They even did the full genome sequencing and assembly of SCN in 2020, last year.