Calcmoolator is a handy online calculator collection
Calcmoolator is a very neat collection of task-specific calculators, with a cute bovine mascot. The site provides an enormous number of calculators, like "Auto Lease vs. Buy", "Tip Calculator" (shown above), "Canadian Mortage", and even "Cost of a Baby" (a weird one, but it must come in handy for some people).
The site looks very clean. Once you enter all of the variables and hit Calculate you get a human-readable result, such as "One baby will cost you $4,284.00 for one year". You can then click the Summary tab and get a tabular result ("One Year Costs: $4,284.00").
Since the site is so numerically-oriented, it should not come as a surprise that they offer a Calculator Stats page which shows a list of all of the calculators they offer, sorted by popularity, with a nice pie-chart on top. You can also search for a specific calculator from that page.
One notable omission is any sort of health-related calculation. I mean, calories vs. food, how long it would take me to lose X pounds, anything like that – not on this site.
There are also iPhone and iPad versions of the site, which is good because it seems to be a very handy tool when you're out and about.