Sondigo, a PC Audio/sound card company, recently made the jumpup to Drupal 5.1 at www.sondigo.com. Previously we had been running Drupal 4.6 since the company's inception last year.
Module-wise, we're using Legal for user sign-ups, as well as TinyMCE so that non-web developers at the company can add content to the site easily. Other than that, we are quite well served by the capabilities in core.
It took about three months, but it's finally up and running. My brain is fried, but it's finally up and running. Every once and a while I twitch uncontrollably... but it's finally up and running.
And worth it, I think. It was worth the trouble it took to get done.
A little background:
I publish a webcomic called Help Desk. I've been doing it since 1996, and for most of that time it was all static pages. When I joined Keenspot (a webcomics collective) in 2000 I added some templating tools they developed that allowed the static pages to be built automatically every night, but it was still not a particularly dynamic system, and I always thought that was a shame. I'd always wanted the site to be sort of like Slashdot -- post a comic and let people comment on it if they wanted to.
Previously, this was an html site with 3 wordpress installs (blog and 2 podcasts), 1 phpBB and 1 phplist mailer. Now, it's simply one drupal site! Wow, what will I do with all the time?