One lesson I learned from Eric Ries is how powerful an incremental, reactive approach to testing can be. It's really hard to balance resources between development and testing, especially as a starving startup, but if you build tests to catch errors that have actually happened, you know you're focused on high-priority issues.
We started Jetpac with a very minimal deployment process with few automated checks, but two stages so we could eye-ball the test environment before pushing it to the final set of live servers. Yesterday that manual process finally failed after we accidentally pushed a completely broken build to the main site and took it down for a few minutes. That gave me a strong reason to add the first automatic checking to our deployment scripts to make sure we couldn't push to production if the test environment wasn't responsive.
To start with I just wanted something very basic that will catch glaring errors that stop our Ruby app from running entirely, since that was what actually happened and they're pretty simple to detect. To do that, I wrote a short Ruby script that can be invoked from the command line and will spot empty responses, 404's and other obvious problems with a URL. We invoke it like this in our deployment bash script, after calling Capistrano to do the actual push:
if [ $? -gt 0 ]; then
echo "Deployment not allowed, test server is not responding"
It will print out information to stderr about any problems it encountered, and handles both http and https URLs. I'd imagine as our needs grow we'll turn to something more complex like Capybara, but for now this simple script is a very quick and easy way of catching a lot of common problems.