As the web has evolved so has the need for vaild XHTML / CSS designs which no longer use tables to control the structure of the design but use DIV's and CSS. You can find a number of free Joomla templates but you can also find some really high quality CSS templates which are free ( released under the GPL licence ) and you could convert the designs to Joomla.

Here are just some of the web sites offering free css templates

OSWD - Offers a wide range of free css templates submitted by the user community.

Freecsstemplates.org - Very cool site offering some of the best CSS templates on the net.

Solucija - Also has some great CSS templates which you can use.

Arcsin.se - Another great site offering some very cool CSS templates for free.