LiveJournal
Support/tutorials:
everything_lj (http://community.livejournal.com/everything_lj/)
overridehelp (http://community.livejournal.com/overridehelp)
test_sommeil (http://test-sommeil.livejournal.com)
howto (http://community.livejournal.com/howto)

Layouts:
spiralbound_lj (http://community.livejournal.com/spiralbound_lj)
freelayouts (http://community.livejournal.com/freelayouts)
premade_ljs (http://community.livejournal.com/premade_ljs)
CreateBlog LJ Layouts (http://createblog.com/layouts/index.php?type_id=2)
LJLayouts (http://ljlayouts.org)

Blogger
Support/tutorials:
Blogger Forum (http://www.bloggerforum.com/modules/newbb/)
Google Group: Blogger Help (http://groups.google.ms/group/blogger-help)

Layouts:
Blogger Templates (I) (http://blogger-templates.blogspot.com)
Blogger Templates (II) (http://www.bloggertemplates.org)
Noipo Blogger Templates (http://blogtemplates.noipo.org)

WordPress
Support/tutorials:
WordPress Support (http://wordpress.org/support)
Codex: WordPress Lesons (http://codex.wordpress.org/WordPress_Lessons)
Dissection of a WordPress theme (http://www.urbangiraffe.com/2005/04/12/themeguide1/)

Layouts:
alexking.org: WordPress Styles (http://www.alexking.org/index.php?content=software/wordpress/styles.php)
Codex: WordPress Themes (http://codex.wordpress.org/Using_Themes/Theme_List)