Was offered by Microsoft as their core Web fonts, and freely redistributable, MS no longer offers it free on their site. However, the original license allowed free redistribution, so other places make them available.
Ranked in these QuestionsQuestion Ranking
Pro Excellent legibility
It's easy to distinguish "0" from "O", "a" from "o", and "I" from "l" from "1".
Pro Good font variants
Boldface variant is not too heavy while italic face is not too spindly. Andale Mono contrasts with Source Code Pro, which is very similar in the regular font face.
Con Not pure mono
Bold and italic are too wide, so it's not pure mono in combination with regular font.