What makes for a really usable website? What are the most useful websites and why?

For me, a really usable website can successfully combine aesthetics and function...it looks good AND it works too! I prefer websites that can fit all necessary elements (e.g. navigation bar, tabs, links, etc.) onto one page that doesn't have to be scrolled in order to find any particular element. I don't know if it's a matter of color, form or layout, but I like websites that seem to make important information effortlessly stick out from other less important content.

A really usable website should be able to be accessed and utilized by any number of typical or atypical users, regardless of topic. I hate websites that take forever to load (e.g. unnecessary images, faulty code, etc.). I also dislike websites that have too much "stuff." If there's an overabundance of text, images (static, not so much--too many flashing or moving images irritate me), or other visual elements, it makes the website less usable because of the distraction, frustration, and annoyance factors.

The most useful websites are ones that can keep you connected and active for as long as possible. You go to that website for a particular reason--to answer a question, to look up information--but, even after satisfying your initial motive, you stick around and check out what's new on the site. For example, I think imdb.com (Internet Movie Database) is very useful. I can go on to look up a quote, actor, movie, etc. and, more often than not, I end up spending another 10 or 15 minutes reading up on current media happenings and poking around to see what's new on the site.

On the flip side of all of this, a useful website knows when to relinquish control over its' user. For example, askjeeves.com gives you the opportunity to inquire about any number of topics and then sends you away to actually obtain the information. I think the bottom line for the most useful websites is attention to the getting and keeping of audience.