Douglas Fils in his blogs has put a list of recommendations for Groovy/Grails developers. The original post (5 Groovy / Grails recommendations) can be read online. His key recommendations are:
web sites : the groovy and grails websites (I think putting these on the list seems a bit unecessary. Except for some, most well known and well adopted open source projects usually talk a lot about their own stuff. It that surpirsing?)
Podcasts: Sven Haiges has an interesting Grails podcast. Very cool! (Podcast is soon becoming the new blog, you don't have one of your own, you probably don't exist yet :) )
RSS Feeds: Mentions three top aggregators on all things groovy. I can vouch for groovyblogs.org. I get all my groovy news there.
IDE(s): Says IntelliJ IDEA rocks! Also says Netbeans 6.1 may be a possible choice. Netbeans has been jumping on every single badwagon passing even close to the JVM. They are feature packed as on date. What they need now are some real users (outside of Sun Microsystems)
Books: Dierk Koenig's Groovy in Action he says is a "wondeful resource". Sure, I think the book is useful. The only problem is that like most Manning (in Action) books its a fiction novel in style -- takes 696 pages to explain things which I think should take 1/2 the number of pages.