I'd love to have some new maps, flags, sounds, and such but certainly don't want to spend days going around to various websites acquiring them and then sorting through for quality control. So is there a single, "essential" collection of custom content for download anywhere?

It's difficult to create an collection of 'must-have' maps because many maps are scheme-specific and no-one wants to end up with loads of maps for a scheme they don't play.

There is a WormsPack, but it's quite old now. It's hosted at www.blamethepixel.com , look in the left column.

*Shamless plug for the big map pack* See my sig :P

That only contains large maps though, the pack linked to by Muzer is pretty good, but does include a hell of a lot of stuff, much of which you may need to sort through either to organise it or just remove what you don't want.

[UFP]Ghost (http://forum.team17.co.uk/member.php?u=33232) has collected the most worms files to my knowledge.
He also included the complete Worms Pack (which was compiled by Rastamahata over two years ago).

[UFP]Ghost once seeded a torrent with only the speechbanks from his collection. They already were over 1GB in size.
AFAIK, he has several more gigabytes of Worms stuff. IIRC, he even downloaded all the maps from wmdb.org at some point.