I've been having a bit of fun with this and finding out about my sense of time and tempo.
Tap for Beats Per Minute BPM
I've been using it in two main ways.
1) Measuring tempo near the start and near the end of various performances, including my own, to see how how closely the tempo has been maintained.
2) Trying to "hear in my head" tunes and measuring the tempo, then playing the recording and seeing how close I was to the actual tempo on the track.
It's also good for just practising keeping a constant tempo. Simple, but informative, imo. Try it and see what you think.
(I thought this was the best place to put this as it didn't seem to fit in "Technique").