I've been rather fond of Adobe's Source Code Pro[1][2]. Notably, it comes in weights all the way down to ExtraLight. My typical setup these days uses ExtraLight in iTerm2 on retina displays. Depending on the display (and sometimes the font renderer), I'll play with combinations of weight and size to find the best fit for a given installation.

[1] http://adobe-fonts.github.io/source-code-pro/

[2] https://github.com/adobe-fonts/source-code-pro

(going offtopic) Seems everyone has a different taste. I prefer medium-weight fonts, Courier is way too light for me. Do you work on a Mac BTW? I think there might be some massive rendering difference between Macs and other OSes. You can tell by reading people's blogs. Some authors use super narrow, gray-on-white font, which is generally unreadable on Windows, but I think it may look just fine on macbooks.

Personally, depending on editor (each editor has slightly different rendering, spacing, interline etc.), I'm using DejaVu Sans Mono or Lucida Console.

I dislike Consolas, because it's too blurry for me; so much that I generally immediately delete it from my OS installation, in order for it to not show up when I'm reading coding blogs.

Indeed, rendering makes a huge difference between OS'es. Personally, I find the Mac (and iOS, those are equal I think) rendering quite nice, the Windows rendering reasonable, and the Linux rendering (at least in gnome, don't know about other wm's?) just ugly, but that's just my opinion. :P

Source Code Pro and Input Mono both have light and extra light weights.

https://github.com/adobe-fonts/source-code-pro

http://input.fontbureau.com/

+1 for Input font from Font Bureau but with anti-aliasing enabled. I've been happy with Monaco without anti-aliasing enabled.

Consider https://larsenwork.com/monoid/

I'm a big fan of TheSansMono - but its not exactly cheap (I think it was about $120 for the "office" group which I bought).

The characters are pleasing, easy to read, and really I just find them nice. http://www.lucasfonts.com/fonts/thesansmono/thesansmono-offi...

Inconsolata-dz looks pretty lightweight in dark color scheme on a 4k monitor. Not as light on normal monitors.

http://nodnod.net/2009/feb/12/adding-straight-single-and-dou...

Office Code Pro Light has been pretty great: https://github.com/nathco/Office-Code-Pro

Try Dina, it's like Terminus but comes out even more crisp. I get giddy every time I open an emacs window.

Dina only goes up to 10pt, which is too small for me. 14pt Terminus is great.