I have been using Terro Liquid drops for about a year now, and it is absolutely the best ant killer ever. This product comes in various forms like clear ant trap trays, outdoor stakes, and my favorite - no-frills drops from a clear plastic bottle.
I like the clear drops best because it gives me the flexibility to put the bait right in the middle of an active ant trail. For indoor ant problems, I just put a piece of painter's tape across the and trail, and drop a couple of drops where the ants will find it. The stuff works instantly. Ants find the bait, eat it, and sound the alarm at the mound that a delicious meal has been found. Within a few minutes the bait is surrounded by dozens of ants, these ants take the bait back to the queen, and the mound is neutralized after a single treatment. Once the ants are gone, I peel up the painter's tape with any remaining bait, and clean the area with a household cleaner.
I uploaded a couple of pictures from the bait I set out today in my kitchen. You can see the circles of ants around a couple of drops of this bait on painter's tape. In one picture you can see tiny bits of debris around the bait. Believe it or not, this is food that the ants were carrying back to the mound that was dropped when they encountered the bait. In other words, the ants love the Terro so much, they drop the crumbs they found under my oven!
I have never seen this bait fail. Buy it, try it, you will not be disappointed!
Hints:
1. When ants find the clear liquid bait, they will immediately begin their feast. They will stick their heads into the bait and lie motionless while they eat their fill. Often this lasts for 5-15 minutes. Once they are full, they will return to the mound and signal that food has been found. Within about 30 minutes of the initial discovery, hundreds of ants will be eating the bait.
2. If you are not a bug person, be forewarned: This bait is going to attract hundreds of ants. The best thing to do is to leave them alone and let them eat. The goal here is to let the workers carry the bait back to the mound to kill the queen. Depending on the size of the mound, I have seen this bait work in as little as 2 hours, and as long as 12 hours. For big ant problems you will have to replenish the bait as the ants consume it. Eventually no ants will be eating and you know that you are done with treatment.