The CLISP Common Lisp implementation is a good general-purpose interpreter licensed under the terms of the GNU GPLv2. Though not as popular as SBCL, it sports an intuitive user interface, a byte-code compiler and an easy-to-use debugger.
It also includes various useful extensions to the official ANSI standard, such as foreign function calls and a socket interface. (Note however that using implementation-specific features binds one to the GPL.)
Download one of the packages or pre-built binaries.
Installing on Linux
On Ubuntu/Debian-based systems, you can install CLISP from the repositories with the following command:
apt-get install clisp
On Arch-based systems, use:
pacman -S clisp
On Fedora (>= 18, if you have an old version of Fedora, replace
dnf install clisp
emerge -auvDN dev-lisp/clisp
Note that you will probably need admin privileges to install any software on your
computer (e.g. by using