Instant Domain Search
Ah, Ajax. How fond I am of thee. Instant Domain Search is what it sounds like: As you type in the entry field, it shows you in real time whether the .com, .net, and .org domain names are available. If the domain is available it presents you with a number of registrar options (I wish it showed prices next to each), and if it's not available it gives you options for back-ordering, Alexa query, and so on. Instant Domain Search also has a handy feature that lets you save searches (in a cookie) by pressing Ctrl+S. Of course, Instant Domain Search is getting referral bonuses for every domain registered through it, but I can't fault it for making some money. I think for brainstorming the name of your next web startup it could definitely come in handy.
