Slight resemblance to : Bog Pimpernel, to which it is related, but Bog Pimpernel is five times smaller, has pink flowers with rounded petals and grows in peaty bogs, not in gardens.
Some similarities to : Yellow Pimpernel (to which it is also related) but yellow pimpernel has petals that are (when not closed in cloudy weather) more pointed, have more space between petals, and are not cup-shaped. Yellow Pimpernel also has styles that stick out much more prominently upwards from the flowers than does Creeping Jenny.
Creeping Jenny is more likely to be seen on garden walls or creeping over paths and is popular with some gardeners, although given half a chance it will slowly creep, by stealth, over the whole garden only revealing its true extent when in flower. It readily takes root along the nodes.
No one else seems to comment on the fact that it has square stems!