Retina Macbook Pro. You can run it at a variety of resolutions that all look very sharp - 1920 x 1200 is very usable, but you can bump it way up to stupid high resolutions if you want screen space.

I've found it's awesome because I can effectively go from web browsing on a 1440 x 900 laptop to working on the equivalent of an iMac (2560 x 1440) or even higher (2880 x 1800) without losing sharpness.

If you then want extra space, you can plugin in an external monitor, but to be honest, I ended up selling the two externals I used to use (they were 1080p, and seemed small and blurry in comparison).

Using OSX, you also get the benefits that it's very unixy/linuxy, so you get bash, ssh, openssl, grep, sed, ruby, python, php, apache etc as part of the system. With Xcode you get other tools like git. Configuring an acceptable dev environment for web dev can be done without downloading anything extra. (Of course, to get an awesome dev environment does require a bit more work, but arguably much less than the equivalent on Windows).

Currently using a Thinkpad X230i with Arch Linux + DWM. The ultra-portability is great and it's been a great machine for all around development.

I Bought it "used" for $500. (i3, 500gb ssd, 4gb ram)

http://www.ktgee.net/post/49423737148/thinkpad-guide

If you really need screen estate, get an external monitor. I've used a 15 inch MBP since 2008 and before today I couldn't have imagined downgrading. I bought a used 11 inch MBA off gumtree today and have surprisingly found it more than adequate for the job. In particular I'm enjoying the prospect of truly mobile computing.

Personally, I think the most ideal web development machine is the 13 inch Dell XPS Developer Edition with Ubuntu.

I'm using an HP Chromebook 14 running Ubuntu. I've got an external monitor for when I need it - hardly ever. Current resolution is 1776 x 999.

Some of the best things about it are the built in 3G modem with 2 years free internet, how light it is, how cheap it was even though it really looks very nice, and how quickly it boots.

I use a Macbook Air, sometimes connected to a Dell 24" monitor. My next laptop will be Macbook Pro Retina.

I cannot fail MacBook Pro 13" with Retina display. The 15" variant is a bit too bulky for travelling. I also use Dell 24" external monitor when working from the office.

Macbook pro with Retina, and a Dell 24" monitor.