Clip Studio Paint Pro (Non-Localised Manga Studio 5 [Superior]) 70% off - $21

This is a well-known Illustration software favoured by a lot of artists. Version 5 features some innovative features which causes it to outperform Photoshop in some cases. If you're interested in digital illustration I highly recommend having a look at the different features or even just downloading the trial.

Clip Studio is the ORIGINAL software so by buying through the developers website you're directly supporting them. It also gets updates and fixes more frequently than Manga Studio. (Also it's not a "Manga" software, it's full fledged illustration software so don't be misled by the name. Though there are manga-specific features especially in EX you will be getting a product along the lines of Photoshop, Sai etc.)

They have regular sales on this product which discount it quite heavily. The last was in September and this current sale is the last of 2015. There's also a 60% discount on EX and a discount on upgrading the product to EX.

For the price it's an excellent product.