Here are 4 tools/concepts every online startup should be using. For many this is basic stuff - but when I was starting out, I found out the hard way.
Backup everything! You're gonna need it. Task.fm recently surged, this resulted in the server constantly crashing and a hundred other problems. Thank god I had been taking daily backups. Make sure you back up server settings, databases and the actual app. Also don't forget about things like your company blog - make sure this is included in any backup. Also download, or store your backups remotely. If your server crashes what good is backup stored on the server.
This is a little different to the standard backup. Basically, a snapshot backup takes an image of the server. Think of it as a restore point. Every time I push a new version of the app, I take a snapshot backup (takes about 4 hours) which is stored remotely. If anything goes wrong, I can restore the server right before I made any changes.
Make sure your developers don't overwrite each others work! Surprisingly this happens all the time. Subversion (apparently all the cool kids use Git these days) keeps track of all the changes devs make and keeps a handy copy of each version of every file . We host the Subversion remotely (thanks Beanstalk), which means if the server crashes, I still have a copy of the app for easy restoration.
Real Time Stats
I don't want to find out why the server was peaking yesterday, today! I need this data in real time. From a marketing perspective, being able to see where people are talking about you in realtime is a killer advantage. Real-Time analytics for large sites is now possible and a reality thanks to services such as Clicky