This app is great, it takes the place of all other image searches! I don't know what you use for your initial list now-a-days (an update function might be a nice add), but it would help others to know what they do.
First, GazoPa and Cydral are no more, and E-Hentai has changed their API or something, so it no longer works.
Google, TinEye, and Bing are well known.
The real stars are the ones that do something other than search:
ImgOps is a gateway to a plethora of picture editing programs.
Regex gives the metadata on a photo, way more than I ever realized existed!
The others all search specific areas that Google & Co. might miss:
KarmaDecay = Reddit
SauceNAO = Manga and Anime
IQDB = Anime and Manga
Ascii2D = Japan
Baidu = China
Yandex = Russia.
I do hope that Yahoo becomes available, to join the big three image search engines.