After upgrading to Leopard I switched to Miro from Transmission, an otherwise excellent application which seemed to trigger several runaway Spotlight MD processes, possibly because Spotlight in 10.5.5 may have trouble with incompletely downloaded Transmission torrent files. After losing almost a full day's worth of work time debugging the issue, I stopped using Transmission and haven't had any trouble since.

Except for the key omission that Transmission doesn't automatically download torrent files, I would rather be using it than Miro, with its mysterious interface, or Vuze, which is a slow, bloated, complicated Java app that wasn't stable on my system and required a reinstallation before it functioned. On Tiger, Transmission functioned flawlessly. I'll try it again on Leopard in a few months.