First of all I found some sites with real estate listings. The ones I found/used/tried to use were: Χρυσή Ευκαιρία, Rento, Spitogatos and aggelies ta nea.

Each one though has it own benefits and problems, apart from some who only have problems.
Aggelies Ta Nea:
pros
None. I can’t find anything innovative about this site.
Cons
i) It has very few listings of places to rent in the areas I liked (downtown Athens).
ii) It is full of listings by real estates agents who ask you as payment one full rent if they manage to find you a house.
iii) There’s no map showing where each house is.
iv) There are pics of very very few houses in the listings.

Spitogatos:
Pros:
This site has a really neat feature, price per square meter. It’s quite nice to have the site calculate it for you.
Cons:
i) It has very few listings of places to rent in the areas I liked (downtown Athens).
ii) It’s default drop down price filtering boxes are a bit weird. It goes from 150->200->300->500->750>1000 Euros. So if I choose a price range of 300-500 euros I get a url like this:

http://www.spitogatos.gr/gr/search/results/residential/rent/r100/m2011m/nd/all/300/500/nd/85/nd/nd/nd/nd/nd/nd/nd/nd/all/rankingScore_desc

If I change it to:

http://www.spitogatos.gr/gr/search/results/residential/rent/r100/m2011m/nd/all/350/450/nd/85/nd/nd/nd/nd/nd/nd/nd/nd/all/rankingScore_desc

I get exactly what I wanted.
Having drop down boxes might be fine for some people, but they don’t let me be as specific as I would like. A form to fill the price range by hand would be a lot more useful for me.
iii) There’s no map showing where each house is.

Rento:
Pros:
i) Rento is the most innovative site I found. Every house listing is on google maps and you can access its details by just clicking on a house.
ii) It also features a VERY innovative search bar. You actually type a sentence about the house you would like and it searches for it.
iii) Each listing has pictures
iv) You can contact the owner by email
v) There’s an option to note each listing you like so you get something like “bookmarks”.

Cons:
i) It has very few listings of places to rent in the areas I liked (downtown Athens).
ii) The search bar did not have a negation clause. You can’t search for “not something”. So since I didn’t want a ground flour house, I couldn’t filter them out.
iii) The search bar would sometimes filter more than you asked for. If I searched for a price range of 350-450 and got some houses, then if I search for a 40-60 sq. meters I got some others. If I searched for both the price range and the sq. meters I got very very few results.
iv) Many of the listings were quite outdated. Places had been rent weeks ago and the listings were still on the site. (I guess that’s a problem with real estate sites…owners don’t tell the sites whether the house has been sold/rented when that happends).
v) There’s no way to see the most recently placed listings.

The awkward thing about Rento was that I met the people who manage it in a Ruby meeting in Athens one week after I got the house. They were aware of these problems and they said that they have already corrected them and will push their changes to the site very soon. I sure hope so because the site is definitely worth it.

One suggestion for rento would be to have an option to export as kml the “bookmarked” houses.

Χρυσή Ευκαιρία:
Pros:
i) Many many houses listed.
ii) The filtering for the search works very well.

Cons:
i) Very few pics of the houses (if any)
ii) Not every house is listed on a map
iii) In order to get the owner’s telephone you have to send an sms, or call a number and pay some amount of money.
iv) Not every house has an address listed.

I ended up using Χρυσή Ευκαιρία due to it’s massive database with listed houses. I tried to use rento and spitogatos but I just couldn’t find what I wanted. (Maybe I’ll get luckier when I’ll try to move to a new house.)

I then created an unlisted google map called “new houses” and started placing marks on the houses from Χρυσή Ευκαιρία that I liked, sorted by date of last update, and were placed on a map in the site. Then I started calling the owners of the rest to find out where they were. If they were in a place that I liked I made an appointment to go and check the house.
I placed all the appointments at the “TagToDo List” application for my android.
Unfortunately I couldn’t use the “My maps Editor” by Google on my android due to some bug it stopped connecting to google maps. It would be really useful to have this app because I could have all the places I placed on “new houses” and have them with me. Instead I had to print the maps with the marks on them.

Finally in order to walk around the city and not get lost I used the Rmaps application. It’s so much better than the standard google maps because you can get many different maps, and with the addition of GPS Status you can copy paste your exact location to any notes applications you might be using on android to track new houses you find while walking.