For a DIY solution some Draytek routers will load share (notice I didn't say balance) across a couple of ADSL connections.
http://www.draytek.co.uk/products/vigor2820.html
What to watch out for with load sharing/balancing across multiple links is the method used to achieve it. If it's just routers at either end load sharing whilst you will end up with sum total bandwidth of the links used, EACH individual session will be limited to the maximum speed of the individual link it's routed over. So for example if you have a need for all the total bandwidth with one FTP session then this isn't the method for you and MLPPP would be a better choice, however if you have got lots of things going on at once then it will be OK.
One of the rules of the telecoms world is whatever bandwidth you put in it will all get used up pretty quickly and you well end up looking again at increasing bandwidth. So should you be using your existing bandwidth in a better manner, for example setting up some firewall rules to ban or limit streaming video etc..