I've downloaded and successfully installed Xilinx ISE Web Pack 14.4 on Ubuntu 12.10 although I'm unable to launch it.
Well: it's UNIX, so there are multiple ways to do it.
One of them is this
Some preparations (needed only once)
$ cd .../14.4/ISE_DS
$ chmod u+x *.csh *.sh
Execute the correct settings script you can find in the root of the installation
$ /path_to_the_installation_dir/14.4/ISE_DS/settings64.sh
For a none cshell system with 64 bits.
Run a Xilinx tool
$ /path_to_the_installation_dir/14.4/ISE_DS/ISE/bin/lin64/ise
$ /path_to_the_installation_dir/14.4/ISE_DS/ISE/bin/lin64/coregen
$ /path_to_the_installation_dir/14.4/ISE_DS/EDK/bin/lin64/xps
$ /path_to_the_installation_dir/14.4/ISE_DS/EDK/bin/lin64/xsdk
If you want a better system I advise you to take a look at the TCL version of the Modules tool.
How to setup your Unix environment to point to the correct licenses can be found in installation document of Xilinx.