Not today, actually on Friday, but it's all been put into action today.
A new 44 tooth chainring, SRAM PC1 chain and Sturmey-Archer 22T Freewheel, for my commuter.
I tend to abuse the drive train on the commuter horribly, so it doesn't last as long as it should. The freewheel only lasted a couple of years, just over 10000 miles. The chainring and chain were older, and since the total cost of the above was £35, that's not too bad at under 0.35p per mile (or alternatively it is under 9p for a days commute 1, which certainly beats the Tube and Tram!)
Footnote [1] OK, I haven't counted wear and tear on the frame, brake blocks, tyres, inner tubes, bar tape, racks, panniers, and cycling clothes, all of which need periodic replacement too. That total would still be a lot less than the £7-50 if I used public transport.