This list was posted a while ago, I just edited it and add some new ideas/inspirations!

Tutorial sites:

• http://tutorialsandmore.tumblr.com/
• http://theblog-diaries.tumblr.com/
• http://theamazingtutorials.tumblr.com/
• http://tumblrtutoriial.tumblr.com/tutorials

Simple themes:

• http://atonalsthemes.tumblr.com/
• http://www.smpldesign.co.uk/
• http://balloonies.tumblr.com/tagged/theme
• http://bamdesign.tumblr.com/
• http://gyapo.tumblr.com/tagged/theme
• http://themesbybluntstatements.tumblr.com/
• http://olderthemes.tumblr.com/
• http://newerthemes.tumblr.com/
• http://sleepythemes.tumblr.com/

• http://tit.tumblr.com/one-column-tumblr-themes
• http://themes.themaxdavis.com/
• http://www.tumblr.com/themes/by/mirnah
• http://pretty-glamorous.tumblr.com/mythemes
• http://sleeplessthemes.tumblr.com/tagged/themes
• http://vanilla-chai.tumblr.com/
• http://paulywesley.tumblr.com/tagged/mythemes

Challenges:

• http://heckyeahtumblrchallenges.tumblr.com/
• http://30daychallengearchive.tumblr.com/

Dashboard themes:

• http://userstyles.org/styles/browse/tumblr.com

(Source: footprints-inthesand)