There are many plants and animals named for the naturalist Charles Darwin, such as Darwin's Frog (Rhinoderma darwinii), but which were named by him? I'm finding it difficult to find such a list.
I don't know if these are his earliest descriptions but Darwin did describe several species of Planaria, such as Planaria vaginuloides, P. oceania, plus a new genus, Diplanaria in 1844.
Darwin, C. R. 1844. Brief descriptions of several terrestrial planariæ, and of some remarkable marine species, with an account of their habits. Annals and Magazine of Natural History 14 (October): 241-251, 1 plate.
Perhaps his best known taxonomic work is on barnacles (Cirripedia). He described several species in his monograph first monograph on the group.
The first monograph was followed by three other monographs on living and fossil Cirripedia.
I believe that most of his writings after publication of On the Origin of Species focused on expanding his evolutionary ideas rather than on descriptions of new species.
You can read nearly all of Darwin's writings online at http://darwin-online.org.uk. With careful searching you can find all (or nearly all) species descriptions.
I've discovered that searching for Darwin on the Encyclopedia of Life (EOL) appears to prioritise in its search results those species named by Darwin rather than for him. The first page of results includes many barnacle species (as noted by 3cat). The first five species are:
There are 370 results, but some of them are not species level (e.g. Chthamalidae), and some other entries are mixed in (e.g. S.C. Darwin and S.P.Darwin), and later results include, for example, Nemoria darwiniata, so it's hard to quantify the results.
I might try writing an SQL query for the downloaded Catalogue of Life, as its online interface doesn't seem to allow the relevant query.