Well, OK, not an accessory as such, but it didn't fit in anywhere else really.
These have been around a while. They're basically rectangular planting troughs, with a couple of bars attached along the side for locking a bike too. The weight of the soil should make it pretty immovable, but AIUI, they can be bolted down too for added security.
A plantlock is a more attractive piece of street furniture than a normal type of stand, but how does it perform?
Well, I came across one near the Giggling Sausage in Southwark last week. The bar is easy to lock to, with enough of a gap between the bar and the planter to fit a lock into. The height is useful to get front wheel & frame locked together.
Downsides? Well, it's quite low, so I can imagine the bike falling/being pushed over, which would be bad. And the locking bar itself seems pretty narrow. I believe it's a rod rather than a tube, as on a Sheffield, so there's a decent amount of metal there, but it doesn't seem so secure.