Baudelaire and pretty much every romantic poet/philosopher. Absinthe and hash.

Their standards weren't all that high when it came to psychoactives.

Absinthe has since been all but proven to be completely devoid of a psychedelic effect - good absinthe never really had any appreciable amount of thujone in it; the fabled "absinthe high" being the synergistic effect of alcohol and the neuroprotective and refreshing herbal essences - an extremely clear drunkenness that is significantly lighter on drowsiness and body load. (See here, I can also second this as I enjoy some real French absinthe time to time. The closest thing I can liken it to is drinking alcohol and coffee together.)

As for hashish, one can clearly get a serious trip out of it (or well, out of any form of cannabis) if one is willing to accept the body load that comes with the dosage, there is no question about that... but I'm wondering if romantic poets ever had that much. They didn't really have access to Skunk strains I guess. The regular cannabis high is nice and otherworldly enough so that a romantic poet may exaggerate it to, well, a romantic magnitude.

Null24 already mentioned Baudelaire. Baudelaire was a visitor to the "Club des Haschischins" organized by Théophile Gautier since at least 1846.

In the USA there was Fitz Hugh Ludlow and his book "The Hasheesh Eater" (1857).

Lewis Carroll, of "Alice in Wonderland" fame, apparently "was a fairly heavy cannabis smoker. According to one source, he regularly bought hash oil, which was legal at the time." (source).

There are also claims of several american presidents smoalking (and inhaling!) fine home-grown bud since as early as 1765.