Deluge
I guess all of us have been in the situation when ratio at a certain tracker has to be urgently improved :) All bittorrent clients I know, deluge included, offer very little to help - you can control seeding options either globally or individually for each torrent, so about the only thing you can do is to manually pause other trackers' torrents which is not nice at all - lots of manual operations, other torrents do not seed even if some of the bandwidth is not used.
I suggest adding an option or maybe a plugin to control seeding parameters by tracker: desired ratios (seed all torrents of this tracker until ratio 2), desired absolute upload amounts (prioritize seeding to this tracker until we put +100G there), priorities (this tracker gets at least 60% of bandwidth). I think this feature will be requested and it will make deluge unique. I must admit there are things I don't like about this idea - it puts free trackers even in less advantageous position, but life is a cruel thing.