I have been using Anime Studio and Manga Studio for this, at various times. I use Anime Studio as its the vector graphics based animation package that I use for doing my characters, and I need that same look for the in-game items. Trouble is since its an animation package its not well suited to just doing static graphics. I therefore was using Manga Studio which is my comic drawing software - it gives lovely pen strokes and cel shading as per Manga comics, but unfortunately the resulting look is not really a match for the vector art that comes from Anime Studio.
Hence I have picked up Artboard, for $35 from the AppStore. I would have looked at Illustrator but its very expensive and over-featured for what I want. I also tried Inkscape and that would have been my preference but the performance going through X-Quartz on the Mac was just awful. On bigger drawings that pen lag was just killing me. Artboard is a nice combination of features and for price, and has great performance on my laptop.
Here's some of the things I have done with it so far.
|Alarm Clock to wake our hero up out of bed|
|Room render with Artboard drawings on top|
I particularly like the expert styles - I used the ability to create copies of the stroke and outset each one to create the sort of "bevelled-in" look on the dial face for the clock. This stroke was stacked on top of one of Artboards pre-packaged styles, a glossy black which was just right for the face of the clock.
The point editing mode of Artboard is very good too - you can quickly go from shape manipulation mode over to path mode and back again, just by double tapping on the object. In Inkscape its not so easy to go between treating an object as a path and as a shape.
I have come across a few bugs, but the authors of Artboard - one of whom is an Australian software engineer - seem to be quite responsive, and they're producing new releases so it looks hopeful that these will get dealt with soon.
If you're looking for a simple but relatively powerful and fun graphics editor for your game work I recommend giving Artboard a try.