I produce a system that can do this. It's called Broadbond.

You can bond several ADSL lines, even from independent providers, and it will deliver the combined upstream and downstream bandwidth of the two. All traffic is load balanced across the two lines and can also be transparently compressed. The throughput of the lines is automatically measured to determine the optimal load balancing. Differences in latency on the two lines are compensated for.

The catch (there's always a catch!) is that you need to have a partnering system co-located with an ISP to handle the far end of the tunnel -- although I can also provide this if you would prefer.

The system is available as a software package that you can license to run on Linux or OpenBSD and also pre-installed and pre-configured on a couple of small embedded Linux boxes -- very low power (under 5W), no moving parts, good for up to 90Mbit/sec.

I bond two ADSL lines to my office, 4.4Mbit and 9.6Mbit, and I get around 13.5Mbit on file transfers.

If you're interested, contact me (details on the broadbond.org web page).