Measuring download speed of web pages is a big thing for me - and there are extremely few tools that can be used to reliably gather both the download time of individual pages once and twice (to simulate the effect of internal browser caches - and intermediate proxies) - and do this at regular intervals to find the effect of varying server load throughout the day.

Episodes seems like a good tool to solve part of this requirement :

Episodes: a Framework for Measuring
Web Page Load Times
Steve Souders
Google
July 2008
Abstract
Interest in web performance is growing rapidly. Web companies are trying to use speed as a distinguishing feature. At the same time, web pages have more content than ever before which makes for a slower page. Ajax helps reduce the number of roundtrips required for a web application, but today's alternatives for measuring web performance don't work well for Web 2.0 apps.
What's needed is a way to measure web page load times that works for Web 2.0, is easy for web developers to adopt and maintain, can be leveraged by web metrics service providers, generates data usable by web development tools, and provides context to browsers so they can give better feedback to users about their experience.

[From Episodes: a Framework for Measuring Web Page Load Times]