They're mostly gimmicky because they were all made for a middleware contest. Even their gimmickiness doesn't really go against the goals of the contest, the primary one being "to encourage Ruby developers to explore the possibilities".

In the end, even if very few end up being actually useful, there will be plenty of good short examples using a variety of different Rack tricks.

That said. Rack::Cache. And I wrote myself for one site that displays some extra info if a request comes from certain IPs (i.e. mine).

If I had to guess, I think Rack::Cache is very popular - http://tomayko.com/src/rack-cache/ - and not so gimmicky :)

I use Rack::Flash (http://github.com/nakajima/rack-flash) in a bunch of Sinatra apps.

I am using Rack::Deflater to gzip output, as there is no easy way to do this with lighttpd.