Hi,

Is there any browser that allows me to change the agent string manually?

This is usefull, for example, to simulate being a crawler bot.

I have a totally database driven site full of parameterized pages, and I'm making a routine to extract the information to present to crawlers in a friendly format.

I am not sure about browsers, but there are browser proxies which do what you are talking about. For example, The Proxomitron.

You need to modify some keys in registry. Here is the tutorial

http://www.winguides.com/registry/display.php/799/

I think Opera can do this.

Its really easy to do with PHP :)

Ther is an extension for Firefox too that does this
http://www.chrispederick.com/work/firefox/useragentswitcher/