Discovered two forum scripts today.
IceBB looks a lot like MyBB and IPB. Seriously. (Homepage here.)
Quicksilver Forum seems to me, style wise at least, to be reminicent of some old phpbb themes. Hard to read, small text, not pleasing to look at. In fact, it reminds me of Phorum. (Of course, most of my problems with it can be dealt with in the css file.)