need an external monitor for my 13" Air

There's the Apple Thunderbolt display, of course. Nice thing there is that it also powers your Macbook (with a MagSafe2 adapter).

Other than that, I'd suggest any monitor with DisplayPort interface, to prevent needing a MDP-DVI adapter.

 

Consider the following
Resolution, nothing less than 1080 in my opinion
Colour space, depending on what you do
Connectability, does it come with the right ports and leads? If not you might have to find another chunk of cash.
We use NEC screens at work which have built in USB hubs which is nice. A couple of the guys hook up their MBP's and have a full size keyboard and mouse.

 

I recommend IPS displays. If going with 24" then a resolution of 1920x1200 is a must. If going with 27" then you need to go 2560 x 1440. Do not buy a 27" display @ 1920x1080 as it will be awful. I'm not going to recommend a display simply because the models change almost daily and there are too many. Figure out your budget and read some reviews.

 

Dell U2412M.

24" - Check
IPS - Check
DisplayPort - Check.

 

U2711 ftw

 

Another vote for Dell U2412M. Works fine with a MiniDisplayPort to DisplayPort cable (some displays don't). If you don't want to go all the way to 27" I'd seriously recommend this.

For 27", I'd go with the Apple Thunderbolt Display - I use the similar Cinema Display and it is fantastic - and the extra ports would be particularly helpful on a n Air. The potential deal-breakers for that are whether you like glossy displays (personally I think it is swings and roundabouts) and whether you want to be able to use it with anything other than a post-2011 Mac.

There's a lot of speculation that Apple might release a new display "real soon now" with the new iMac-style antiglare panel and USB3 - but AFAIK there's absolutely no hard evidence of if and when that will happen.

 

Also use Dell 2412 ... Text isn't so good but colour is, lots of inputs, adjustable height, swivel for landscape or portrait, and very cheap compared with Apple TB.

I didn't want to invest a lot, as I think screen technology is going to improve soon.

----------

The text sharpness is about the same as my 2007 iMac but the imac's gloss screen makes everything seem more dynamic than the Dell's extreme matte screen.