I would like to have a dual screen setup with my 21" iMac, but I'd like to avoid the expensive 27" official monitor from Apple.
Is there a specific 21" screen that would fit well in size, resolution, disposition and colors with the iMac screen?
My experience is that if you are doing pro level design where color matching is critical you won't be happy with a LCD panel that isn't of equivalent quality and also covered with glass. Look for a IPS LCD panel that has similar wide viewing angles since you won't want to move your head 15 inches sideways to get the same angle when comparing a critical design.
That being said, for casual use the iMac will drive pretty much any old or inexpensive you care to place next to the iMac. Many developers are happy to have a second window for email, chat, tailing server logs, twitter, messaging and don't really care if the second display actually matches the iMac since different content is being displayed on each display.
Sorry to offer a mushy - wiggly answer, but after seeing many years of people that were happy with even an old CRT next to a new iMac and others that go batty with even a really nice perfect LCD match (say by doing the work and finding that Dell sources the same IPS display as Apple for a near perfect match of hardware) and still being irritated that the appearance is different due to a piece of glass used on the iMac.
It comes down to your personal preference. Trust your gut - if you will look at an inexpensive monitor and get to work, glad you have more screen space - save as much money as you can stand. Even if you think you will notice the difference. Imagine a pile of 20 dollar bills (or in your case a pile of Euro notes) sitting there on your desk six months later down the road. Money you don't spend is still yours to spend later.
I am a programmer and I have tried many monitors out there and nothing even comes close to the quality of apple cinema displays. I know that they are more expensive but you can get a 20" apple cinema display on ebay for around $150. Definitely recommend apple displays.