I am using Scientific Linux. So is there any stuff that can be safely removed from disk except
One of the easiest and safest thing to do is to clear the package manager cache. You did not specify which operating system or distribution you use so I will give examples for the mainstream distributions:
Most, if not all, other package managers in other distributions have a similar functionality.
Another thing you can relatively easy do is delete old kernel versions. Most distributions keep the old kernels installed when installing a new kernel.
Here is a guide how to remove old kernels for debian and fedora: http://www.cyberciti.biz/faq/proper-way-to-remove-old-linux-kernels/
If you want to reclaim disk space, the best way is to uninstall applications you don't need. List the biggest applications that are installed and check if you really need them. Since you didn't specify your operating system here's how to do it on Debian based systems:
aptitude search '~i' --sort installsize -F '%I %p'
Another approach is to delete user data that's not required. To find big files or directories, you can use a tool like baobab.
All or much in /var/tmp can probably be removed. Much in various /var/cache-directories - eg. previously downloaded apt-packages - can be removed. In the users home-directories, thumbnails generated by various file-browsers (eg. .thumbnails) can be removed. Also the caches of various web-browsers can be removed.
Beside getting rid of stuff you don't use, disk analyzing tools are also helpful because they help you find stuff you don't even think about. Maybe an application forgets to clean up its old logs, maybe old backups that you forgot about and don't need anymore.
The main difference is, instead of looking for stuff that you can delete, these tools show you the stuff that takes the most space, and then you decide whether that space is legitimate or wasted.