I hesitate to recommend a voting system for the exhibition, because the stuff that shows up is varied in the amount of time spent on it, whether it's just a tech demo or an art project, and so forth. Above all, what one person may find amazing may be unimpressive to another. But some sort of way to show approval to the author, or find ones that are most-liked, would be helpful.

With this in mind, the most simple solution would be to add only a positive voting system, much like digg has. If you like a piece in the exhibition, give it the thumbs-up and the number of thumbs will appear by the name. It serves the purpose well, and is a nice way to tell the author "that's cool."

If each exhibition entry furthermore had a comment system attached to it (or a corresponding thread were made for it in some forum), viewers could give more in-depth comments and discuss the code in depth.

I wouldn't be in favour of a rating system, for a couple of reasons.

It would change the nature of the network links from "I made this" to "What do you think".  I think a place for "I made this" is useful, and the place for "What do you think" is here.

It's totally appropriate to use this forum to start a discussion about your work - posting extra context for the processing community, letting people know how it was made, asking how it might be improved, etc.

Sites like del.icio.us and digg are great for finding out if people like your stuff, but they're hard to emulate.

Digg's system isn't positive only - the comments section is used to say 'no digg' fairly often.  And the comments are generally inane and throwaway, not high quality criticism or general encouragement.

I think CodeTree is better placed to do this sort of thing for small self-contained applets, and that bigger applets should be presented in a way that allows them to get feedback from the web at large.

Check out sites like GeneratorX, Data Is Nature, Info Esthetics, Rhizome, Interactive Architecture, Pixel Sumo etc.  Take a look at the work they're posting, and if yours is similar then let them know about it. Even Boing Boing posts Processing sketches from time to time.  And, of course, most of the contributors to Processing Blogs are likely to pick up on things posted to the exhibition if they are worth talking about.  Or you could add comments to your sketch page and get direct feedback there.

Further afield, Technorati and Ice Rocket are sometimes enlightening to find who's linking to your stuff, but they are very weblog-centric.