Location based alarms
I wrote anorak: http://anorak.sourceforge.net/ to solve the train problem -- on dark nights knowing when you are close to your station. anorak just uses cell tower ids for this purpose, so you don't even need GPS, but locations are only as good as the cell tower density.
I moved house and started to commute by bike and found anorak less useful (entering details for new cell towers as you pedal along is likely to get you a Darwin award).
So, I moved on to GPS based stuff: http://launchpad.net/johnjohn
johnjohn needs more work + there are some neat things that could be done to combine ideas from anorak.