The Matrix powertools plugin is probably the single most handy one for me, ever since they dramatically decreased the choices on multi-city bookings.

A fairly common booking pattern for me is a flight GLA/EDI down to LON in the evening, then a morning flight out to the final destination. With the old BA site, this was very easy to book because it gave all the possible connections. With the new functionality, it's impossible because it will only list a very small number of options, invariably focused on the morning flights down, none of which I want. It also seems incapable of restricting the offers to a specific LON airport. The plugin saves the day and makes it simple to price up the actual flights I want without needing a call to BA.

Perhaps fortunately for the sake of my wallet [1], the plugin is desktop only AFAIK, so I can't use it when on mobile only.

[1] Being away somewhere warm, with good company, food and wine does seem to have alarming effect on my willingness to book something on a whim!