Silk is a solid set, but it's becoming extremely noticeable when it's used. Now it really seems jarring when I see it on a site: it looks skimpy.

It's great for placeholders, but if you can get professional design work done, do it. Don't skip on expenses if it means looking more unique.

For icons I've been using the Crystal set by Everaldo, released under LGPL.

http://www.everaldo.com/crystal/

I have a particular dislike for that icon set. Things that are shiny are too shiny and things that aren’t shiny look flat. It also has a bad case of Apple envy; but is nowhere near up to the OS X icons’ standard.

Agreed. There are better LGPL sets. We mostly use Nuvola by David Vignoni, though I've grown a little tired of its cartoonish looks, and David has since done better work (but none as complete as Nuvola...we paid for development of a couple dozen of the Nuvola set, so it's complete in ways that no other is for our purposes).

Do you know of any more sets like this?

The thing about the Apple icons is that they're all surprisingly well thought-out. Despite the general joke, they've very rarely overshiny and that gives them a really nice, solid feel. Things like the default Leopard folder and the TextEdit icon are entirely non-shiny. So when people aim to out-Apple Apple, it feels overdone, like this icon set in particular.