Here's an example:
* (let ((a 42))
#?"foo: \xC4\N{Latin capital letter U with diaeresis}\nbar: ${a}")
"foo: ÄÜ
bar: 42"
It was written by Edi Weitz and now maintained by community at Github.
CL-INTERPOL can also be used for HTML templating: http://paste.lisp.org/display/146518