I use monit, but honestly on my 256mb VPS (non-mission critical) it threw a ton of false positives. Granted, I configured it wrong. The web UI is pretty neat, albeit basic, and if you set it up to allow restarting processes that can be handy in a pinch. Just be sure to check your configs, otherwise Apache might be restarted every time a rails app gets launched due to memory constraints. ;)

As a disclaimer, I am definitely a neophyte sysadmin so I'm certainly not using the tools correctly. I ended up turning monit on just for checking if my processes are running instead of checking for memory issues. Seems to be running fine, and I haven't heard a peep out of it since I changed the passenger module configs.