I’m familiar with load balancing.. but Is it possible to actually bond multiple DSL lines together? I hear of ways to bond using MLPPP but that requires support from an ISP. Is there a way to actually bond without support from my ISP, or use say a cable modem and a DSL line together for faster speed / diversity?
Need more information about your specific setup. Are you trying to load-balance between multiple WAN connections off the same ISP, or from different ISPs? Is your ISP willing to share BGP style routing information with you? And how much money do you have for the solution at hand.
The industry preferred solution would be to purchase a small to mid range Cisco router and configure it to use BGP routing on each of the WAN interfaces. The router would aggregate the routes from each ISP connection and route packets through the least used / shortest length connection available.
That being said you mentioned DSL / Cable which leads me to believe your talking SOHO with a sub -$200 network gateway device and using two different ISPs. The short answer is "not the way your thinking" but I'll go into the longer answer also.
Routers send packets to the next destination based on a routing table. SOHO devices tend to route everything internally just fine but use a default next hop route pointing to your ISP's gateway for everything not internal. They do this by using NAT / MASQ and playing with the packet headers. When dealing with two separate ISP connections you end up with routing conflicts as you can't have two default routes. One will always be preferred over the other.
This leaves you with the choice of working with the ISP's so they can redistribute their routing tables to your gateway router and let your router decide which path is the best (most ISP's won't go for this) or you manually specify different traffic down different paths. The routing device you use will determine what kind of control you have over your traffic, if its fairly sophisticated (Linux based distro with a real FW / Routing solution) then you can specify source / destination along with protocol and ports to redirect the traffic.
Post back with more info and we can delve further into this rabit hole.
Load balancing is an option but doesn't really help single file downloads. I'm also not going to get BGP support from my any local ISP nor do I have an ASN. I'm looking at a couple of technologies that claim to use GRE tunnels to solve the problem. vunity says they give me some sort of box that forms GRE tunnels over multiple DSL's and then combines the GRE tunnels at their data center...
In theory, this would mean that I could bond anything with anything.. I could bond a bunch of DSL connections with a cable modem and get automatic failover and the total aggregated speed of all links.. what do you think?