MAC Fluidline is described as a "gel" but to me it's more like a paste. You can use various brushes for it (which I'll get to in a moment) to create different looks. It comes in quite a few colours and MAC also occassionally releases limited edition shades. It lasts through an 8 hour work day without any fading and comes off easily with a baby wipe at night. One pot generally lasts me 6 months of frequent use.
MAC #212 ($27.50): For simply lining the eyelid without a wing or lining your waterline. This isn't my favourite but I thought I'd throw it in here.
Sephora #17 ($14): For doing a thin flick.
Sephora #23 ($20): For doing medium to thin flicks. I'd recommend this to beginners as it's the easiest to use. Benefit also sells a brush like this.
Cheap paint brush (Galeria #2 from Curry's Art Supplies): I think this was $5 and it works very similarily to the Sephora #17 for a fraction of the price.
The only other gel eyeliner I've tried is the Maybelline one which I hated. I want to try the L'Oreal, Essence, and Sleek versions to see if they live up to MAC.
Availability: MAC, $18