Earlier today (~4:45 PM EST) I went on Stack Overflow to look up some information on Bash. Everything worked fine, and all text rendered normally.
I went on later (~7:45 PM EST) to check on something else only to find highly intrusive ads. This included bubble ads (see below), generated text links (also below), and full-page invisible links (where you click on the page and it loads another page in the background).
My first thought was a rogue extension, but Chrome (only browser installed) didn't have anything unusual. To be safe, I disabled everything and tried again. Still ads. I installed a script blocker (ScriptSafe for Chrome), which on first visit disabled scripts for Stack Exchange. This removed ads. Enabling scripts for the domain brought them back.
Developer Console seems to reveal the source of these popups is
I ran scans. Spybot S&D returned nothing. MBAM found four cookies. Windows Defender returned nothing. HitmanPro found tracking cookies. AdwCleaner returned nothing.
I did a more thorough trace, and it oddly enough appears to be coming from Google Analytics. I traced backwards from the first occurrence of DNSUnlocker.
http://istatic.eshopcomp.com/fo/ec/gteamrs.js?subid=64001&bname=DNSUnlocker&blink=http%3A%2F%2Fwww.dnsunlocker.com > http://m64.dnsqa.me/QualityCheck/ga.js : 10 http://m64.dnsqa.me/QualityCheck/ga.js > http://www.google-analytics.com/analytics.js : 45 http://meta.stackexchange.com/questions/267846/adzerk-and-dns-unblocker-intrusive-ads > http://www.google-analytics.com/analytics.js : 1374,1375