Questionable sources are sources that fail to provide acknowledgement to the origin of the information, authors are either crowd-sourced or not experts in the field of the topic, any blogs without source citations, opinion pieces, and/or interviews with people who aren’t experts/have credibility.
Blogs: one way to evaluate if your source is a blog or an opinion piece is to examine the URL: if there is /blog/ or /opinion/ within the URL, ask your instructor for approval.
Wikipedia
about.com
answers.com
ask.com
ehow.com
idebate.org
procon.org
quote banks/clearing houses
buzzle.com
salon.com
dailymailuk.com
more.com
fusion.net
dodo.com
study.com
explainthatstuff.com
sheknows.com
livestrong.com
huffingtonpost.com
onlymyhealth.com
bustle.com
governing.com
debatewise.com
infoplease.com
enotes.com
whowhatwhy.com
wordpress.com
suite101.com
redorbit.com
buzzfeed.com
intelwire.com
articlesbase.com
wisegeek.com
hubpages
ezine.com
dailybeast.com
PETA
Note: While these sources are considered questionable they might be a starting point for finding credible sources. For example, you can not cite Wikipedia as a credible source but, exploring an entry's reference list may lead you to a credible one.