Are there any services that will monitor a web site and report if it gets hacked and defaced? Which are reputable and recommended? How does the service notify you when your site is having problems?
Google Webmaster Tools will notify you of malware. It will show a message in the console, and I think it will email you as well. I am not sure about how swiftly they do so though.
We use a service like that from serviceuptime.com. Its been working well enough for us not to change. We do occasionally have issues with getting it to send out SMS alerts reliably and promptly.
We had something internal we hacked together before we used this outfit. One day the power went down and the generators at the hosting company failed and then the battery backup gave out. Everything went down and we didn't know. Lesson learned, always monitor up-time from an external location(s). It's also great for detecting DNS configuration changes/issues.
I'd be happy to hear of other services that might be better.
Sucuri offers a monitoring system which will notify you via e-mail when a page is changed. It also has specific notices for malware infections that are being run through a website. They can also monitor DNS records, WHOIS information, and SSL certificates in relation to your domain and website. Their bare-bones basic service is free and their paid packages are reasonable for larger websites.