Would you like how your web application performance behaves across longer time periods? Now it's possible with site-uptime.net! We've just added visualisation of statistics:
Ops! We had some performance problems 2010-03 .. 2010-07 (twitter.com, BTW). Looks like they were fixed in next months.
We are going to allow store and show day-by-day visualisations as well to give you feedback in shorter amount of time.