Compiz has a widget layer - don’t really know what it does though.
I believe AWN and cairo-dock have rudimentary widget engines built in.
The best standalone widget system I’ve used is screenlets. I think development is now moving into Universal Applets instead, but screenlets is more reliable at the moment.
There was talk (on an unofficial level at least) of merging some of these projects, or some aspects of them, into gnome. The push to Gnome 3, and the rapidly shifting UI, may have ruled that out.
There’s also adesklets, which is low on system resources. I think they’re both older, but I’m not sure.
I get the feeling the current gnome 3 implementation doesn’t really allow for much modularity - but given time it’ll probably loosen up. Or I’ll just use XFCE instead.
I have a feeling that will be one of the things that gets added in a 3.x release. I’m fairly certain it will not be 3.0. If anything, they’ll probably find someway (if they are smart) to reuse the plasma widgets or some other common format. However, someone else mentioned screenlets or some other system might get merged into Gnome, which would not be too surprising.
I think one of the reasons official Gnome is holding off is because it’s not something that is really enterprise worthy and secondly they would want it to integrate into Gnome’s strict “look-and-feel” standards in terms of usability.