(Click to Enlarge)
The Rashkey was invented by Oleg Raschkov in 1999 and it's a Rotational/Rearrangement puzzle. It comes in three versions: the Rashkey 1, which is the one I have, the Rashkey 2 and the Rashkey 3. They differ in difficulty with the 1 being the easiest and the 3, the hardest.
The puzzle is comprised by two overlapping disks with centers separated by a radius in length. The version that I have has three colors, with predominantly yellow and red and blue for each disk center pieces. You maneuver it by rotating it at 90º at a time. One thing that I was surprised to find out when I first got it, was that it's much smaller than I was expecting it to be, at about 7,5cm (2.95").
(Click to Enlarge) - Scrambled
What makes the puzzle easy to solve is the abundance of yellow pieces (25), contrasting with only four red and four blue pieces. Since each ring shares two pieces of its four center ones with the other ring, the number of possible positions is much smaller (44.100), compared with 4.961.250 in the Rashkey 2 and 13.335.840.000 in the Rashkey 3. - Numbers taken from Jaap Scherphuis' website.
Solving the puzzle for the first time can still be a little challenging, despite the level of difficulty. It took me about 15 minutes for a first solve. After getting the hang of it, I can solve it under 5 minutes. Besides the basic pattern, you can solve it to other few designs as well.
(Click to Enlarge) - Alternate Patterns
Unfortunately, the Rashkey is not widely available anymore, but you can still find it in a couple of places. I bought mine recently from Milan Vodička, but you can also find the versions 2 and 3 from Hendrik Haak. They occasionally appear on eBay as well, so you might want to check regularly (or save a search on eBay with e-mail alerts). If you like this type of puzzles, you can also check the Arusloky website, as they are widely available. I have one in my collection and they're very interesting too, with many different challenges. The Puzzler is also a similar design, in three versions (see here, here and here), but this is even rarer to find.
(Click to Enlarge) - Backside
Even though the Rashkey 1 is the easiest version from the three, I chose to get it because it was the one that caught my attention the most, for its striking resemblance to two four leaf clovers. While it may be easier than the other two versions, it's very fun to solve nonetheless.