MR Tool
I know that many people here like the ITA software web site for doing travel searches (beta.itasoftware.com).
But I haven't seen anyone mention the advanced search features that make the site really useful for finding MR itineraries. These are documented under "help" -> "extra for experts".
For example, you can type in "ABE::US US US US" in the "from" box and "LON::US US US US" in the "to" box. This will try to find the cheapest itinerary with four USAir flights in each direction [for the default dates in April right now, the best solution is $489].
This removes a lot of pain in finding interesting MRs.
Edited to note that the search is for USAir flights
[This message has been edited by USCheapskate (edited Mar 18, 2004).]