Parodia leninghausii or the Yellow Tower Cactus is an interesting clustering cactus that has round columnar stems that are coated in yellow/light brown spines that are generally harmless.
It produces very attractive, yellow, sliky textured, funnel-shaped flowers in summer and is very popular even though it will take around five years to produce its first flowers.
They start off having a spherical shape but become more columnar as they age and will naturally cluster from the base.
Good for xeriscape and gravel gardens but perhaps best suited to growing in containers where they can be brought indoors in winter. They can withstand a light frost providing they are kept dry - better to keep them protected over winter.