The most legible font I have ever seen – and I wish someone would make an open-source version of it, and a monospace version, along with more glyphs – is Heinemann Special. No two characters look the same, and the font is really pleasant to read. No [back]slashed-zero, though.
Reading through their license it is very much open source. It gives the rights to "to use, study, copy, merge, embed, modify, redistribute, and sell modified and unmodified copies" and thus covers the requirements of open source.
