Couple more to link.

http://iqdb.harry.lu/ - e621 and e621 alone
http://saucenao.com/ - e621, other boorus like gelbooru, pixiv, deviantart, anime and doujins
https://www.iqdb.org/ - Japanese artwork boorus
https://images.google.com/ - Search engine indexed sites
https://www.tineye.com/ - Same as google, if google fails

Some of these services do also keep deleted posts samples, so you should get results even with those. However keep in mind that deleted posts when services crawling was malfunctioning or before the service was established, aren't in the results, so image could've been uploaded here but it will not show using those services at that point.

Then there's also MD5 searching. e621 and inkbunny supports searching with this, so if the image looks like that it could be of original quality, this is worth a shot. Sometimes MD5 is part of the filename or URL, which can help if e.g. image seems clearly downscaled but has hash of original still as filename. Formats of sites are listed here.

Like single cell said, before the image reverse searching was a thing, we have a site where everything is at least hopefully well tagged, so sometimes simply searching with most obvious tags usually gets the result. If you didn't get educated in elementary school how to use search engines to your advantage, remember to click that [color=yellow](Search Options)[/color] next to search field.