what do you mean by "cool?" I am something of an expert on the one-liner tees since my BF has lately been obsessed with starting a company that makes them and he has showed me all the other sites. so those would be: rockstartees, bustedtees, palmercash,t-shirthell, dusty brand... let me know if you need more...
MY favorite one though, is pinkspike.net. she doesn't have slogans, but these really pretty screen-printed graphics, and the cuts of the tees are kind of different, more interesting than generic t-shirt shape. last time I checked the site, though, she only had a few styles, I'm not sure what's going on there. HTH!
I'm a fan of the tees from Smoking Lily. They aren't slogans, but very cool designs. You pick a tee style and color and a design and they make it for you. They use American Apparel tees.
I love the Smoking Lily site!! I forgot all about it. Thanks, luckylily. The gallery is new I think. The storefront is great looking. I would love to go to the store. I feel like I am missing out on a lot of cool stuff they don't have on their site.