As seen in article 19 (a platform game), I had came up with a text file format which I used to create levels.
I was about to start coding a level editor when I came on this editor : http://www.mapeditor.org/ .
This is freeware and it seems active enough.
Note : a nice thread here around possible tile editors.
I decided to use it with my current game project (a platform).
I choose the xml format (tmx format). See an example here : level .
I then had to code a method to parse the xml file.
In the process I also completed the multi-tiles to one block trick to avoid the edge-catching bug.
I also managed the texture tiling.
A class to create level in a farseer world will come in a next article.
Here below a screenshot of a level made with Tiled.
Here below a video playing this level.