<p>I've been looking at ways people test their apps in order decide where to do caching or apply some extra engineering effort, and so far httperf and a simple sesslog have been quite helpful.</p>

<p>What tools and tricks did you apply on your projects?</p>

I use httperf for a high level view of performance.
Rails has a performance script built in, that uses the ruby-prof gem to analyse calls deep within the Rails stack. There is an awesome
Railscast
on
Request Profiling
using this technique.
NewRelic
have some seriously cool analysis tools that give near real-time data.
They just made it a "Lite" version available for free.