Craftool L-bar style stitching groover:
Free for the price of shipping. USPS only.
I used this tool for a long time, and recently replaced the cutter. But I ended up trying one of the pen-style versions, where the L-shaped bar is the guide, rather than the cutter, and I find I like that style better, since I can pull the guide entirely off and use it like a gum tool.
I remember seeing Marcell Mrsan use one of this style to make welt in a video.