I've used the interpolate_fancy plugin fairly extensively to make conditional elements in my flavours so they appear only where I want them to appear. It's a really useful plugin once you get the hang of it.

Apparently Kyo Nagashima at hail2u thinks it's a great plugin, too, and has started converting it to JavaScript so the principles can be applied to any website. A lot of the syntax is exactly the same, too.

<?foo eq="123">display if "foo" is equal 123</?>
<?foo ne="123">display if "foo" is not equal than 123</?>
<?foo lt="123">display if "foo" is less than 123</?>
<?foo gt="123">display if "foo" is greater than 123</?>
<?foo like="123">display if "foo" includes 123</?>
<?foo unlike="123">display if "foo" does not include 123</?>
<?!foo>display if "foo" is not defined</?>
<?foo>display if "foo" is defined</?>
<foo/> display "foo"

I know Blosxom has spawned many children for the CGI, but I think this is the first derivative of a plugin. Good work, Nagashima-san!