The flagship of all online prop replicating, is of course The Replica Prop Forums:
Within this massive community of dedicated, nay, obsessed fans you’ll find help, inspiration, awesome projects and generally enough to keep your fantastical ideas obtainable.
The 405th Division is a halo costuming community, which also has TONS of info on pepakura building in general.
The Dented Helmet is centered around building the Boba Fett helmet, but has loads of star wars costume and prop info, as well as stuff about process and technique.
Also, Club Obi Wan is where to go for all things Indiana Jones. I’m talking Grail diary, fedora, guns, maps… everything! It’s a private community so you must register, but the wealth of knowledge inside rivals any ark or holy chalice.
Another brilliant source is Adam Savage’s Tested. Along with helpers Will and Norm (and the ever elusive Jamie) these guys bring so much to the table. Tech reviews, one day builds, electronics mapping, sculpting, lego(!). Their site has paid access to juicy premium content, but their youtube page has enough content to keep you inspired for months.
Also it was through Tested that I came across two greats in the business:
Volpin Props is the work of Harrison Krix, an extremely talented prop builder, who went from amateur to professional in no time. His blog details almost his every build, with even more detail on his Flickr. This man is worth your time.
And lastly, Frank Ippolito is a brilliant make up/costume/sculpture dude. His work is beautiful and he’s now done countless tutorials for the Tested guys (including a life-sized Rancor costume). He also does a regular creature podcast which I’ve been quite enjoying.