There are 2 online tools I find myself using quite often and whose, in my opinion, should be in any utility belt of any web-designer and web-master.
WebPagetest, a site where it test the load time of your page and analyzes what loads, how long it loads, and even gives suggestion on how to improve it.
And of course the most important one: Browsershots. Ever wondered how your website looks in Opera 8? Or IE 6? Or any other dozen browsers out there? You don’t have to install 10 browsers to find it out. This webservice takes care of it for you by taking screenshots in each browser and uploading said screenshots upon completion.