I left Courier new behind quiet a while ago since you cannot distinquish 0OilI. On my way to my new favorite there have been a lot of disappointments, namely Annonymous(just not readable at 14px..), Lucia Sans Console(messes up the indentation).
Im using 16px fonts at the moment, which helps me to focus on one part of the program and spares me to get close to the monitor, to check the spelling.
After throwing away Lucia Console i went on my final search and settled with Bitstream Vera.
16px looks rather lager compared to this 10px lines, but if your got enought resolution, its no problem.