At heart, a backup is just a copy of your files. As a result, it is possible to use tools to perform backups other than the traditional backup applications. Tools that automatically synchronize data between two locations, suchas the Unix rsync command, are commonly used as backup tools. Such tools can be automated to run at certain times using cron. They are not usually the most effective solution and they can present problems when backing up database files, including Open Directory data on a Mac OS X Server (which is a big reason to spend the money to buy a real backup tool).
Other options include disk duplicating tools such as Apple’s Disk Utility (and the various front-ends to its duplicating features; for example, Carbon Copy Cloner and Super Duper). Again, cron can be used to automate disk duplication, making it a backup option. These methods sidestep the problems inherent in backing up databases and can reasonably be used as solutions for Mac OS X Server. They even offer an easier complete restore process than most backup applications (just boot from an alternate startup disk and use Disk Utility to restore them from either a disk or a disk image).
The problem is that you cannot do incremental or differential backups with this approach. You have to effectively perform a full backup every time. Also, this will make it difficult, although not impossible, to maintain multiple generations of backups.