I have a pretty yellow cactus blooming today, Parodia schlosseri (Parodia erubescens), which is native to Uruguay. I think that's what it is, anyway - it didn't come with a tag. Whatever it is, it's pretty.
The bright red stigma in the center is so striking.
Each flower lasts a couple days, opening in the morning when it gets direct sunlight and closing in the evening.