Can you recommand me a software for doing this job? I shoot using Sigma 8mm lens and I would like to be able to obtain a normal image out of that. The camera is canon 550D.
Try Panorama Tools - free and very effective: http://panotools.sourceforge.net/
Useful thread here: http://photo.net/digital-darkroom-forum/00CBe8
There is a friendly interface to it called PTGUI.
Ken Rockwell wrote a review of DxO Optics that illustrates converting fisheye to rectilinear photos. The most impressive examples are the kitchen ones about 3/4 of the way down the page. I've used DxO and it's pretty good at this as long as you have a body/lens combination DxO has profiled.
Not a cheap solution, but certainly worth considering if you need the ultra-wide angle, but don't care for the distortion.
There are many other less comprehensive posts and of course DxO's own site. Google "DxO +architecture +fisheye".