Some of the following are more overtly skeptical or sciency, and some or more generic content, but all are maintained by members of our wider skeptical community.

http://beskeptical.tumblr.com/
http://the11dimensions.tumblr.com
http://troythulu.tumblr.com/
http://r3v.tumblr.com/
http://unbridledconfusion.tumblr.com/
http://questionall.tumblr.com
http://skepticaltruebeliever.tumblr.com/
http://helvetebrann.tumblr.com/
http://acuddlephish.tumblr.com/
http://saudihominid.tumblr.com/
http://godlessgeekblog.tumblr.com/
http://undergroundskeptic.tumblr.com/
http://ofpaperandponies.tumblr.com/
http://the-cogitator.tumblr.com/
http://virtualds.org/
http://collegeskeptic.tumblr.com/
http://reasonable-doubt.tumblr.com/
http://esceptico.tumblr.com/
http://memkaos.tumblr.com/
http://obligategeekotroph.tumblr.com/
http://randomskeptic.tumblr.com/
http://briandgregory.tumblr.com/
http://sittingonthewashbowl.tumblr.com/
http://fuckyeahskepticism.tumblr.com/
http://ignatius-m.tumblr.com/
http://doubtingmarcus.tumblr.com/
http://tokenskeptic.tumblr.com/
http://noisyastronomer.tumblr.com/
http://rosesandrocketships.tumblr.com/
http://living-as-if.firstsigns.org.uk/
http://chelseaepperson.tumblr.com/
http://scottsigler.tumblr.com/
http://fatoneinthemiddle.tumblr.com/
http://debgod.tumblr.com/
http://snakeoilskeptic.tumblr.com/
http://procrastinatrx.tumblr.com/
http://membracid.tumblr.com/
http://benetherington.com/
http://abaldwin360.tumblr.com/
http://skepticfreethought.tumblr.com/
http://fuckyeahmythbusters.tumblr.com/
http://freethinkersalon.tumblr.com/
http://liberatingthought.tumblr.com/
http://sideburnpanda.tumblr.com/
http://rossexton.tumblr.com/
http://www.amateurscientist.org/
http://how-come-i.tumblr.com/
http://fucknopseudoscience.tumblr.com/
http://the11dimensions.tumblr.com/
http://jakelsewhere.tumblr.com/
http://silkyjedi.tumblr.com/
http://questionall.tumblr.com/
http://dispatchesfromexile.tumblr.com/

And since science and skepticism are so closely intertwined, here’s mission-to-mars’s list of the best science blogs on tumblr.

http://14-billion-years-later.tumblr.com/post/6820880890/tumblrs-to-follow-science

I’m also sure I’ve missed some really good skeptic tumblrs, so if you haven’t already checked out the tumblr tracked tags, I recommend the following tags (some are more active than others)