The main vendors are divided into product-only companies and service provider companies. The product-only vendors (Asigra and Avamar) will sell you their stand-alone product to do your own offsite backup, or you can go with one of their service providers to buy their product as a service. Evault, Connected and LiveVault can sell you their service (where they host the vault) or their stand-alone product (where you host the vault). This provides you a nice upgrade path. You can start with the service, but upgrade to the stand-alone product when the per-gigabyte pricing of the service gets a too high.
As to which one to choose, I would listen to the product-specific differences between those five products. Almost every one of them is aiming at a different segment of the market; therefore, only one or two are probably suited to your requirements. Decide which of these two vendors' products best meets your needs, and then decide which reseller/service provider you're going to use. You can always switch service providers if you don't like their service.
This was first published in January 2006