Of catnip, lemon catnip and Caucasian catnip.
Lemon catnip (Nepeta cataria 'Citriodora') looks just likereal catnip (Nepeta cataria). It even shares the same latin name (except for a minor detail like a cultivar name). The difference is in the scent - and the taste.
Catnip smells of catpiss and has an aromatic taste, complete with a small hotness that's missing from other Nepeta species. Some cats get real wild about catnip.
Lemon catnip smells and tastes of lemon. Not quite lemon balm, but pretty close. Cats don't care about lemon catnip at all.
I got some lemon catnip plants a couple years ago, and, being warned that "this will self-seed as profusely as catnip", I planted it away from my normal catnip. This was in a shadier spot, and the lemon catnip has real trouble surviving there - and there's no profuse self-seeding at all.
The problem with my normal catnip is of course that yes, it self-seeds profusely, but so does the Caucasian catnip (Nepeta grandiflora). Which isn't as pretty as catnip, which cats aren't interested in at all, and which is an even worse weed than the normal catnip - it'll pop up everywhere. And then some. So I've been pulling out blue-stemmed seedlings, because a full-grown blue catnip has blue stems, where normal catnip has green ones. And I've been wondering where my normal catnip disappeared to, it's been coming up here and there for years but no longer --
-- ah yes. Nibble on a leaf of those seedlings before you rip them up; if they're bland and boring they're Caucasian catnip, if they have a bite they're proper catnip. And if they by any chance taste of lemon they're lemon catnips. (Course, the uninitiated might think that young nettles (Urtica dioica) and deadnettles (Lamium album) are catnips too, but they'd be wrong.)
Then there's the Siberian catnip (Nepeta sibirica), which is about twice as big as Caucasian catnip, with far larger flowers, but which unfortunately smells bad and flops over everything. So I've been ripping that one up, too, whenever I've tired of its suffocating greenblue lushness.