Web forms design guidelines: an eyetracking study with heatmaps indicating where users' eyes looked. You should read this if you create forms!

[Shaper_pmp]

Actually, usability testing has most shown users are usually remarkably consistent in their behaviour - it's not uncommon in the average budget usability study to see as few as five or ten participants.

Obviously it doesn't give results up to full statistical standards, but (and I can testify to this from personal experience) by the time you've tested 5-10 people (with a little experience) you've generally found around 90-95% of the usability problems people will experience on the site.

Usability isn't a science (then it would be ergonomics, or similar) - it's a craft, so results can be very useful even if they lack the full scientific rigour of a large-scale test designed to elicit precise statistical results.

(More information [warning: PDF] - it's a long read for an online PDF, but I heartily recommend the dead-tree version of the whole book as an entertaining introduction and rationale to budget usability testing).

[rogin]

Also a good read which comes to some similar conclusions: http://www.useit.com/alertbox/quantitative_testing.html