Here you go, any one want to skim it for accuracy:
So, you now have that awesome broadband connection. Now you can listen to streaming music, browse the web, download files, and play games all at the same time. Yeah, it's fast but you want faster. You may have wondered what if you could get two broadband connections and combine them to get twice speed. This is called many things including connection bonding or multi-linking. 
Before you waste your time reading this, this will not increase your speed in games and will add latency.
Increasing Bandwidth
It was originally used for WAN links for corporations before high-speed connections existed. An example of this would be two 4Mb connections combined to make a single 8Mbs connection. You would be able to download at 8Mbs from any site that could keep up. Some dial up ISPs also support multi-linking. This also requires to modems and two phone lines for obvious reasons. So you may wonder why companies and dial up ISP can combine multiple connections together but you can't The reason is because it requires the ISP to reassemble these connections back in to one when it gets to their side. Almost all DSL and other broadband companies do not support this. This is most likely because of the expensive hardware it requires and the increased latency that would make it unusable for gaming.
Redundancy / Load balancing
You can however use multiple connections for load balancing and redundancy. This does not require the ISP's help. Say for instance that you have 100 computers. That is quite a lot to be on one broadband connection. If you could divide those up so they used two connections instead of one, you could help speed up the Internet for every one by lowering the load. Also, if you doing a lot of downloading from multiple sites, it could also help speed it up. You have to remember, if you combine two 3Mbs connections, no single download will be faster than 3Mbs. If you were to have three downloads going at once, one could go at 3Mbs and the two others would share the other 3Mb connection. Why is this good? In addition to reducing the load on a connection, it also provides some redundancy if one connection fails. 
Here are some routers that support load balancing:
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http://www.hawkingtech.com/prodSpec.php?ProdID=101
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http://www.xincom.com/products.html