Someone mentioned Simple Green earlier in this thread, and I wanted to let everyone know, this stuff makes a terrific ant killer for kitchen counters and other sensitive areas. It kills them pretty much on contact.
I've also used Avon's Skin-So-Soft bath oil spray to kill ants. Plus, if you can find the area where the ants are actually coming inside, you can spray SSS right around that area, and that's it for the ants -- they won't cross the stuff. It doesn't take a lot of the product to be effective. I really love it for this use.