Here is a freeware metronome for Windows. It skips beats randomly, to increase the challenge. It has a setting to let you control the skip rate.
http://www.ph.utexas.edu/~ward/metro.exe
No sound card is required -- it uses the system beep.
Some computers seem to have
the system beep missing or turned off, in which case the metronome
is useless.
To get on or off this list see list information at
http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
