For most people, this will be the first monospace font that comes to mind. Courier New. The poor, tired terminal workhorse has been flogged so long and hard to its death that its also hard to be objective about it. However, Courier earns its place in this list for being so ubiquitous. When you're stranded at some foreign PC work terminal that isn't yours but you simply have to code up something, Courier is your only friend.
Monaco comes as Mac OS X's default monospace font. It's decent, but not that pretty to be honest. If I had to describe it as a shape, its like a trapezium with a heavy base and a wonky top. I think I would prefer my fonts to feel like something beautiful and symmetrical. As you can tell, the thing that launched me on this hunt for the right monospace font was also my dissatisfaction with just using the Monaco font. Its not bad though, but I don't feel all that inspired, staring at it.
5. Andale Mono
Andale is another one of those default fonts that comes with Mac OS X, but it is still not too shabby. I find it marginally nicer than Monaco, but my only gripe is that the spacing is too wide and it feels too fat and heavy as a result. Dotted zero, might be good for people who like their fonts spaced out.
4. Droid Sans
The Droid family of fonts, including the serif, was made for use on Android platform. I am a little divided on this one. I appreciate its clean appearance but it has something of that "boring" feeling when I stare at it for too long. Also, no dotted or slash zero, so you may get your O and 0s mixed up.
I like Consolas. Its like a Monaco with a bit less space around it. Crossed zero, round friendly appearance, it was originally designed as a replacement for Courier New, and is one of Microsoft Vista's standard fonts. Don't think its a free font, but I've got Vista and Microsoft Office on my mac so I am thinking it came with either since I only noticed it recently.
Cleancut and stubby, like a homely sort of girl guides biscuit tin, somehow I like Liberation Mono because it makes me feel productive and yet it doesn't feel boring or dull. Dotted zero adds a nice touch. Liberation Mono was made to be the free, open-source replacement to Courier New with very similar metrics.
This is my favourite font of them all. Dainty, crossed zeros, more compact than others. Makes me feel like I'm getting more done. Inconsolata is an opensource font created by Raph Levien and I find it is the most readable font of all... 100 Marks.