Here in Manchester, UK I have recently begun to find Alder Leaf Beetles (Agelastica alni). They are about 1/4 of an inch long and bright, metallic blue. They 'skeletonise' Alder leaves. According to one of my books (pub. 1966) they are, "of rare occurrence in southern England" - not any more they're not!

I'm more into plants than insects and have found quite a few interesting 'aliens' this year, including: Tartar Honeysuckle (Lonicera tartarica), Sweet Spurge (Euphorbia dulcis) and Alchemilla conjuncta - a 'rare' Scottish alpine Lady's Mantle (which may have escaped from someone's rockery). The other evening I found a huge and spectacular plant not yet in flower - I suspect that it is a Himalayan Inula (a relative of Elecampane).