Acanthocalycium spiniflorum 'violacium'

This nice plant from Argentina is a long time favorite among collectors. Easy in culture and ready blooming plant, it produces it's pink flowers reliably over the summer.
There are multiple forms of this species available from commercial seed sellers, with both pink and white flowers and varying spines.

Gymnocalycium spegazzinii

This cactus - Gymnocalycium spegazzinii - is known pore for it's spines than flowers. It's downward looking spines resemble claws, and when plant shivered during drought period they create tight cower over the stem.

Echinocereus subinermis

Echinocereus subinermis is a common in culture plant, native to Sonora state. With low ribs and short spines it can easily be mistaken for Echinopsis when not in flower - the overall plant look is very similar. Flowers of cause will reveal the true identity.
This species flowers yellow, not the most common color among Echinocerei. Flowers are produced reasonably easy.

Gymnocalycium ragonesei

Gymnocalycium ragonesei is naive to salty plains of San Juan, Argentina. This is a species of dry areas, may be the driest of all Gymnocalyciom inhabited lands. It is a slow growing plant with brown stem, and often flat as pancake. In culture it eventually branches, and small clumps are often producing little shows of white blooms.

Parodia lauii

Parodia lauii - one of the relatively resent additions to old genus Parodia. Parodias used to be very popular among growers, and for good reason - they are very colorful plants, with nice flowers too. In recent years  popularity of the genus subsided significantly, the genus simply going out of fashion I'd say. It is really hard to find a good specialized Parodia collection now days.
But plants are same pretty, and with South America's wild places becoming more accessible lots of nice new forms are available in trade. Don't turn away from those beauties!

Cylindropuntia ramoissima flower bud - 'cactella'

Cactus flower is not a true flower, but a meta-morphed sprout, or stem. It is very clearly seen on C. ramoissima example. At early stage of development new flower and new stem are very hard to tell apart, and it is likely they are not different at first, with sprout turning to bud in response to some plant hormones. In my observation, in the greenhouse culture C. ramoissima produces flowers only when in spring time, after the new growth has started, I stop watering plants. That might be an adaptation to grow as much stems as water supply permits this year, and produce flowers once rains has stopped.
The special botanical term 'cactella' was coined to signify this specific feature of flowers in Cactus family. That is, A.Doweld introduced this term as a name for this cactus-specific stem-flower and stem-fruit organ.

Cumulopuntia boliviana

Cumulopuntia boliviana is a plant from high-altitude deserts of Bolivia and Argentina. It forms large clumps in the wild, but in culture it is usually seen as a smaller plant - due to it's slow development.
The pictured plant is a form with especially long spines, over 4 inches long and potentially longer. The plant steadily develops two-three new segments each year.