Installed and enabled:

• DHTML Menus: I love this on my Drupal 5x sites, so glad this is up and running for 6.
• Flash Video: have not tested yet. Noticed this requires the CCK for some related sub-modules it has (FV CCK and FV Views) which doesn't seem to exist yet in the Modules section at Drupal.org Note: check back on status of CCK later.
• Lightbox v2: Ah, I still remember when Drupal 5 came out, I switched over my sites from the 4x series, and then this module broken and remained broken for quite some time. Good to see it's already available for 6! I have not tested this yet though.
• Switchtheme: This gives you the block and saves each users preference in a cookie. Handy module that I use on JasonRiddle.net in particular (Drupal 5x version).