I think I changed the ownership and the rights of the /usr/ folder... And now I can't run some applications... because they tell me that I don't have rights... and the pop-up that asks me some kind of password doesn't appear.

I am using fedora 10.
Any ideas how can I fix this ?

A good start would be to tell us what you did before this happened.

I don't really remember ... what I was trying to do (it was a while ago), but I know I ran chown -R on the /usr folder.

Press the up arrow key in the command prompt. It should show you what commands you ran and hopefully you can see what was typed that included "chown -R"

No chance I've written too much things in the terminal since then.

Isn't there a way to get the default ownership and rights of the folder ?

repo@cannabis:~/download$ ls -l / | grep usr
drwxr-xr-x 13 root root 4096 2009-02-23 18:25 usr
repo@cannabis:~/download$ ls -l /usr
total 216
drwxr-xr-x 2 root root 65536 2009-04-18 19:38 bin
drwxr-xr-x 2 root root 4096 2009-04-07 13:46 games
drwxr-xr-x 47 root root 12288 2009-04-17 09:13 include
drwxr-xr-x 214 root root 69632 2009-04-18 18:00 lib
drwxr-xr-x 3 root root 4096 2009-02-04 12:13 lib64
drwxrwsr-x 11 root staff 4096 2009-02-23 18:35 local
drwx------ 2 root root 16384 2009-02-04 10:39 lost+found
drwxr-xr-x 2 root root 16384 2009-04-18 15:33 sbin
drwxr-xr-x 357 root root 12288 2009-04-18 18:00 share
drwxrwsr-x 8 root src 4096 2009-04-13 18:05 src
drwxr-xr-x 2 root root 4096 2009-02-04 11:04 X11R6
repo@cannabis:~/download$

[mi60@localhost ~]$ ls -l /| grep usr
drwxr-xr-x 14 root root 4096 2009-01-16 22:57 usr
[mi60@localhost ~]$ ls -l /usr
total 304
drwsr-xr-x 2 root root 90112 2009-04-19 13:36 bin
drwxr-xr-x 2 root root 4096 2008-09-06 13:13 etc
drwxr-xr-x 2 root root 4096 2008-09-06 13:13 games
drwxr-xr-x 156 root root 16384 2009-04-16 22:14 include
drwxr-xr-x 3 root root 4096 2009-01-16 22:57 java
drwxr-xr-x 6 root root 4096 2009-04-07 21:30 kerberos
drwxr-xr-x 194 root root 135168 2009-04-18 02:31 lib
drwxr-xr-x 20 root root 12288 2009-04-18 10:31 libexec
drwxr-xr-x 12 root root 4096 2009-01-14 22:56 local
d--------- 2 root root 12288 2009-04-19 13:36 sbin
drwxr-xr-x 355 root root 12288 2009-04-19 13:36 share
drwxr-xr-x 5 root root 4096 2008-12-25 20:14 src
lrwxrwxrwx 1 root root 10 2008-12-25 00:18 tmp -> ../var/tmp



what does this help me?

If you look at /usr/sbin that looks pretty knackered to me. For example mine looks like this.
You might want to change the permissions on sbin and try again.
cheers,
jdk

/usr directory owned by root user and as i have seen your post that you have changed ownership of this directory so revert back the ownership of /usr directory( eg. chown -R root:root path)

try with history, it will result in things that you have done in the terminal

I tried couple of things resulting in totally breaking the system down.(haha I did it, I took linux down ). And now I am on a fresh install :]

Pity ... with rpm based distros you could easily recover
from that little accident using