Liquid Eyeliner by Jesse's Girl in black. I wasn't too excited about trying it, based on my experience with their eye shadow palette (reviewed previously here). But, yanno, a liquid eyeliner can be hard to mess up.
They were touting that this product has a Japanese calligraphy-style brush and that got me intrigued enough to give a try on a day when I didn't really have anywhere to go. And, well, I was greatly surprised by how thin of a line I could easily get with this brush. The first time, it actually took me quite a few brush strokes to get the line to thicken. I've used it a few days in a row now and have gotten used to it where I can draw a thick or thin line at will.
One other advantage of this product, mainly through the brush, is that I could see using it to do tribal markings (which seem to be having a resurgence in the ATS(R), ITS, and Tribal Fusion scene) and having good control and be precise in doing them.
A disadvantage that I found is that, if you mess up, given that the formulation is waterproof, it's actually hard to correct the line as you can't just wipe it off. So if you're not used to working with a liquid eyeliner, this may not be the best product to try your hand at first (there's one by E.L.F. that is like $3 and is easier to manipulate for people new to using liquid eyeliners).
The price is also pretty good, at $6.99. As I mentioned above, there's an E.L.F. liquid eyeliner that you can find for $3 at Target so you could buy that. But I was smitten with the brush enough that I would "splurge" and get the Jesse's Girl product instead due to the precision that you can get. And, well, the waterproof formulation is a nice touch as well.