The Ancestry V2 chip is Ancestry.com's second microarray chip. It is a custom version of the Illumina OmniExpress Plus Genotyping BeadChip. It's their current chip.
The Ancestry V1 chip was Ancestry.com's first microarray chip. It was a standard version of the Illumina OmniExpress chip Genotyping BeadChip.
The 23andMe V4 chip is 23andMe’s fourth microarray chip and is the one now in use. It replaced their V3 chip in November 2013. It is a completely custom Illumina Genotyping BeadChip.
The 23andMe V3 chip was 23andMe's third microarray chip. It replaced their V2 chip in January 2011. It was a custom version of the Illumina OmniExpress Genotyping BeadChip. One of its key features was backwards compatibility with their V2 chip.
The 23andMe V2 chip was 23andMe’s second microarray chip. It is almost surely the Illumina HumanHap550 Genotyping BeadChip.
Kitty Cooper has launched her Chromosome Mapper tool page. This useful tool allows you to color code DNA segments you share with cousins once you have determined your common ancestor. This post covers the basics of using it.