Okay, I'm sure everyone knows that they can use the menu 'Debug>User Agent' to change what Safari presents itself as to a webserver. But many people claim this is a bad idea, and I tend to agree, initially.
The reason they say to not do this is because then web developers will think you are using Internet Explorer or Netscape, and not change their webservers to work with Safari (not that it takes much work!) But what to do when the site you want to use doesn't work with Safari?
Instead of having your User Agent set to Internet Explorer or whatever by default, leave Safari to 'Automatically Chosen' and first load the page with Safari's default user agent (which identifiies itself as Safari). Then after the page has loaded, go to 'Debug>User Agent' and change it. It will only change it for that window (or tab), and will automatically reload the page for you.
Why do this, you may ask. Well, it lets the webserver know that you tried to use Safari, therefore, the developer will hopefully see in their logs that people want to use Safari, but can't because of him. Maybe it'll get him to fix his site.
I use this hint with Citibank's online banking, because while Safari works quite well on the site, it only works when it identifies itself as Internet Explorer! Boo.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040319224845605