OS-tan

The OS-tans are an Internet phenomenon or meme that originated within the Japanese Futaba Channel. 
The personifications as a whole are commonly simply called mascots or mascot characters and as such the Tan suffix itself means nothing outside its role as an honorific and its implications of cuteness. 

The OS-tans are the Moe anthropomorphism/personification of several operating systems by various amateur Japanese artists. The OS-tans are typically depicted as women, with the OS-tans representative of Microsoft Windows operating systems usually depicted as sisters of varying ages. 

The Japanese suffix Tan is a mispronunciation of Chan an informal, intimate, and diminutive honorific suffix for a person used for friends, family, and pets. Normal suffixes including San, Chan, and Kun are also used with OS-tans depending on the character and the speaker's preference or omitted entirely.

 In this case, the mispronunciation is used intentionally to achieve the contrived cute or charming effect that is commonly associated with its use by young children and is also sometimes added to the names of non-mascot used for friends, family, characters.