This has been discussed before in multiple threads but I thought I would mention that for those looking for a hardware solution to provide automatic failover/failback for internet connection, the Cradlepoint devices are a good solution.

You can do one or more ethernet based uplinks (ie: DSL or Cable or FiOS) as well as one or more cellular based uplinks (ie: 3G/4G modems).

Right now there is a reseller selling open-box ARC 1400MBR's (1400W-SP) for $235
Cradlepoint Arc MBR1400 Wireless Router IEEE 802 11n MBR1400W SP | eBay

It's a very good price, especially if you use Sprint as that modem is included. But if you don't use Sprint you can just disregard that modem and still install other modems as needed for your network.

CradlePoint MBR1400 Mission-Critical Branch Router | CradlePoint

Mike

I should mention this device works without cellular as well, if you have two or more hard wired (ethernet) uplinks you want to load balance or failover/failback.

Mike

I have used a Cisco RV016 Multi-WAN VPN Router for several years to manage dual Cable & DSL and it works very well. ($400+ but you can get them for $350). Doesn't have wireless possibility, but allows multiple WAN connections - 16 ports, 2 dedicated Fast Ethernet WAN ports, 1 dedicated Fast Ethernet DMZ port & 13 additional Fast Ethernet ports, five of which may be configured for load balancing or redundant connections to service providers.

They also have the Cisco RV082 Dual WAN VPN Router (8 Port Dual WAN) & Cisco RV042 Dual WAN VPN Router (4 Port Dual WAN) which are slightly cheaper.