I recently dug up this article from last summer in BusinessWeek online. Tagline creation isn't something we're experts in, but when you work with a lot of startup web properties or create your own businesses online, you're bound to need one and hiring an agency at $50,000+ isn't always a wise way to spend your initial capital. From the article:

Top 10 Taglines
1. "Got milk?" (1993, California Milk Processor Board)
2. "Don't leave home without it" (1975, American Express, AXP)
3. "Just do it" (1988, Nike, NKE)
4. "Where's the beef?" (1984, Wendy's)
5. "You're in good hands with Allstate" (1956, Allstate Insurance, ALL)
6. "Think different" (1998, Apple Computer, AAPL)
7. "We try harder" (1962, Avis)
8. "Tastes great, less filling" (1974, Miller Light)
9. "Melts in your mouth, not in your hands" (1954, M&M Candies)
10. "Takes a licking and keeps on ticking" (1956, Timex)

I'd have to agree with Nike, M&Ms and AmEx, and I'd say that #1 is probably a great pick, too. The others, particularly Apple, Timex and Allstate never resonated with me, but I certainly agree that they're all highly recognizable and distinct. But, what makes a tagline great? BusinessWeek asked the experts:

Q: Does a tagline have to be a clever play on words or a pun?
A: Not necessarily. In fact, a tagline that calls attention to itself unnecessarily, like "Look at me, I'm a clever tagline!" isn't good. Being clever for clever's sake will get you into trouble with a tagline.

Q: What separates a good tagline from a bad one?
A: A good tagline is not just a motto or a proverb or a saying or a long-winded mission statement. It's got to be fairly succinct -- usually seven or fewer words.
And it should reflect how your company is positioning itself vs. your competitors. It's taking the brand that you know really well and communicating its value to the world. A great tagline is like the exclamation point at the end of a 30-second elevator pitch.

SEOmoz has never had a tagline, but I think it's something we're going to need to make a bigger push as a brand. Similiarly, unless you're running made-for-AdSense crap sites, you'll eventually want to make an impression with your own brand. Anyone have any favorite taglines they've used successfully in their online business?