A couple of weeks ago I got to try out Speedcurve, which has just launched today.
It's a smooth product which gives both a good high-level view of site performance over time, review the major speed factors (differing browser render time etc) and phases of page load, and index against other competing sites and the wider "ecosystem" of your site's sector. Also, the historic detail is like having a daily request trace (super useful) along with a very-low-resolution archive.org / screengrab (awww!) all of your own.
I've been doing some basic monitoring of site performance using Munin, and what I missed in Speedcurve were the ability to monitor the same set of sites (from previous posts, it's probably obvious that I'm currently obsessing over NZ political party sites in the leadup to the NZ election) and the ability to manually adjust colours (I know, but it just doesn't make sense if the Green Party are orange and the National Party are green).
Very small complaints there - one a beta account limitation (Speedcurve plans and pricing), the other down the bottom of a list of things people want.
The one-off monitoring options will be a really good way to give sites a periodic review ... congrats to Mark on getting it out the door!