Rare plants may or may not be new, in fact, some have been around for decades if not centuries, but remain rare by virtue of their difficult production in a commercial environment. In any case, they will not be available at your local garden centre. You might be able to obtain them at one of the links (mostly high-end mail order nurseries) below under 'Other Cool Plant Sites'.
These are plants that I have not tried before myself, but certiainly hope to in the near future. One that caught my eye was Eupatorium fortunei 'Pink Elegance' at Thierry Delabroye's nursery.
This is the same plantsman that has been introducing the latest Heuchera cultivars, e.g. 'Caramel', 'Tiramisu', 'Pistache', 'Pinot Gris', etc. In any case, I've always been a fan of variegated plants, and a variegated Eupatorium at 1m+ (3-4') would be very cool.
A very cool, but mysteriously rare plant so far in the horticultural industry, is Veronica 'Christy'. This is a hybrid of Veronica austriaca ‘Crater Lake Blue’ and V. prostrata discovered by Christy Hensler in Washington state. It has performed admirably in our trial gardens for a few years now.
Finally, another cool rare variegated plant, is Symphytum × uplandicum 'Axminster Gold'. For whatever reason, this plant is near impossible to produce commercially, but is an outstanding and tremendously colourful addition to the garden.
In my experience, this plant is easy once established and provides very dramatic colour in mid to late-spring. For a variegated plant, it is indeed bold and beautiful.
So, in addition to trying something new or cool, to be really different, try one of these rare plants.
Here's to dirt under your nails, and enjoy.