I'm using this 'clickhappier-archive' account to post archival copies of old MTurk-related userscripts from userscripts-mirror.org (which is itself an archive of the defunct site userscripts.org) and forum posts, which their authors or others haven't re-posted on Greasyfork yet. Some of these may no longer be all that interesting as more recent scripts may do what they do better, but some remain very useful.
This script was archived from this forum post (wasn't posted on userscripts.org).
Script Summary: Alerts when a captcha is encountered. A regular text alert [that] will halt your reloader and also bring the tab into focus.
Note from Clickhappier: See also Chet Manley's MTurk Captcha Alert, which changes the tab/window title and plays a sound when you get a Captcha. This is a simple alternative for those who prefer a basic alert dialog box. Currently I personally use Tjololo's Captcha Counter, which instead provides an alert dialog box on the HIT before a likely Captcha.