Question: I'm setting up an offsite DR facility on a budget. The office has 2 ADSL lines coming in to identical router/modem devices.

I'm looking for a cost effective (cheap) way of joining the two connections so that the network only sees one link to the outside world.

Primarily I'm looking for ADSL failover as opposed to load balancing and bonding but if the solution does all theree then that's fine.

Is anyone aware of a device that does this, or alternatively software that could be installed on a PC with multiple NIC's to manage this sort of thing?

Thanks in advance.

Answer: 
You can install OpenBSD on old PC and use [CARP](http://www.openbsd.org/faq/pf/carp.html) (as Warner mentioned) or [uCARP](http://www.ucarp.org/project/ucarp) (for non-OpenBSD :).

<http://meinit.nl/openbsd-loadbalancing-and-failover-relayd-pf-and-carp>

I wanted to use two ADSL lines a year ago but provider dropped prices so I didn't accomplished this.
I have a [Syswan SW24](http://www.syswan.com/sw24_overview.htm) that I use for WAN failover and it works pretty well. I suspect the firmware has a memory leak because if I leave a bittorrent client running over the weekend, it'll lock up, but otherwise it runs for months without reboot given typical office usage.

[pfsense](http://www.pfsense.com/), among other software options, does this, too.