Here are a few that I use:

• Google Images
• IQDB (for if I think the picture is a possible dup)
• TinEye (Generally for SFW pictures)

Can anybody recommend any more? Any help whatsoever would be greatly appreciated :3

The four located at the bottom left of the menu on a post, plus IQDB and Tin-eye. I also sometimes brute force through status:deleted (context sensitive, usually for helping find sources for people who ask if the image is/was on e621), any of the source accounts provided on the artist page, and have even used normal yahoo searching; all shooting in the dark when trying to source for something really grabs my attention.

With google images, service guesses what is most prominent things on the image and if it can't find the exact same image anywhere, it starts to search for similar images combined with that guess. So for example you search for image which has male furry, google can guess "penis" or "anthromorphic". What you can do is put in all info you do know about, e.g. artist, character, term furry, etc. and it gives much more precise results. This has worked the best when trying to search for content that was altered from original image, like cropped.

Usually manually searching artists and character owners galleries is pretty good way to do things as many sites have either disabled search engine bot access and/or hide NSFW content for logged in users only. Trying to use sites search engine is most likely useless as both search sucks and keywords needed to find the submission usually aren't there.