Ah hah - Azureus Bit Tyrant - from http://bittyrant.cs.washington.edu/

It is pretty good at maximising your download speed. From the FAQ -

Q: Isn't BitTyrant just another leeching client?

No. BitTyrant does not change the amount of data uploaded, just which peers receive that data. Specifically, peers which upload more to you get more of your bandwidth. When all peers use the BitTyrant client as released, performance improves for the entire swarm. The details of this are explained further below. In our paper, we consider situations in which peers use clients which attempt to both maximize performance and conserve upload contribution, but BitTyrant, as released, attempts only to maximize performance.

Q: How is BitTyrant different from existing BitTorrent clients?

BitTorrent differs from existing clients in its selection of which peers to unchoke and send rates to unchoked peers. Suppose your upload capacity is 50 KBps. If you’ve unchoked 5 peers, existing clients will send each peer 10 KBps, independent of the rate each is sending to you. In contrast, BitTyrant will rank all peers by their receive / sent ratios, preferentially unchoking those peers with high ratios. For example, a peer sending data to you at 20 KBps and receiving data from you at 10 KBps will have a ratio of 2, and would be unchoked before unchoking someone uploading at 10 KBps (ratio 1). Further, BitTyrant dynamically adjusts its send rate, giving more data to peers that can and do upload quickly and reducing send rates to others.