It became clear early on in the planning stages of our current project that we were going to need a reliable backup solution. We discussed backing up to another server we already have in the datacenter and backing up to a server in another datacenter. The former was not redundant enough for our liking and the latter would introduce extra costs we could do without so early on.
Enter Amazon S3. You pay for as little as you need and it expands as you do. The S3 docs point to undesigned's PHP5 library using cURL. Perfect. 15 minutes and about 10 lines of code later we were archiving and backing up user content to S3.