If you’re like me and still like to see what your web pages, web apps etc look in an 800×600 browser window when designing and developing this FireFox bookmarklet may come in handy.
Simply create a new bookmark by right clicking on the “Bookmarks Toolbar” in FireFox and create a new Bookmark. Give it a name such as 800×600 and in the location field simply type in:
javascript:top.resizeTo(800,600);
Now, whenever you click the bookmark your window will be resized to 800×600 pixels.

Very Handy. WebDev toolbar also has ablity to to do this.
THanks!
The Quick 800×600 browser resize in FireFox helps a lot.
Thank you Bernard
might want to add: it works in ie as well, of course.