Is real estate plugin for free or not?

Yes, this plugin is for free include source code. Plugin is distributed under GNU General Public License. However, this free software is distributed for a fee. More information can be found here: Selling Free Software

Can I use plugin in more then one site?

Why not, it's GNU GPL – You can use how many copies You want.

I found some bugs and errors, what now?

As licence says:

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

But: You can send me email about them ozana@omdesign.cz. Your comments and suggestions may be included in a future releases.

Image upload not working?