As with any design tool, numerals should be used as part of a conceptual whole -- they connote meaning through quantity or sequence. You can use them to lead people through a list, draw users into a calendar or sequence your navigation.
Know your definitions
It doesn't hurt to know that numerals are the typographic symbols of numbers. So, for example, '5' is the numeral of the number 'five'.
Numerals in navigation
Numerals can be used in short navigation schemes. More than two sections, but fewer than seven, is usually best. A long list can feel overwhelming. Try to use numerals to make your lists feel concise and easily scannable.
Numerals in calendars
The most important dates in calendars are determined by content and context. If you're reviewing ancient wars then the year is probably the most important number. However, if you're producing a calendar design for a present-day scenario, or marking a blog post, the focus should be on a specific day's date, rather than the year.
Numerals in data
Information is increasingly available in metric form in web design, and this can produce some stunning results. Data numerals should be distinguishable and easy to read. Size can be a great way to draw attention to figures, but it's important to retain a visual reference to a label or information source so that numbers remain useful.
Five examples to check out
One of the best data designs I've seen this year is for the business status board application GeckoBoard. These guys use great stats and layouts, clearly labelled, with meaningful use of numerals.
Elegant numerals stack in a short navigation for the design shop Imaginista. The developers have repeated the numbers throughout the horizontal site.
Solo, a freelance project management application, uses numerals throughout its interface, with a great example of decimals on its homepage.
Summer in Tennessee is an awesome tourism site, but I like the numbered subnavigation system most. Numerals stacked with titles and subtitles make for a gorgeous approach.
Using data to draw people into your site can serve as a kind of internal ad, pulling them deeper into your domain. The site for Italian road network Autostrada Pedemontana Lombarda uses numerals in this way, along with chart data.
If you've seen any examples of sites using numerals to great effect, why not mention them in the comments?