<p>Are there any good, cross platform (SBCL and CLISP at the very least) easy to install GUI libraries?</p>

Ltk
is quite popular, very portable, and reasonably well documented through the Tk docs.  Installation on SBCL is as easy as saying:
```
```
(require :asdf-install)
(asdf-install:install :ltk)
```
```
There's also
Cells-Gtk
, which is reported to be quite usable but may have a slightly steeper learning curve because of its reliance on Cells.
EDIT: Note that ASDF-INSTALL is integrated this well with SBCL
only
.  Installing libraries from within other Lisp implementations may prove harder.  (Personally, I always install my libraries from within SBCL and then use them from all implementations.)  Sorry about any confusion this may have caused.