A spread sheet for me! I have a big one that holds all my customer details, contact numbers, cost, frequency, who owes me what; and a remarks column I use for extra information; for example if someone asked me 5 years ago to clean their gutters, I'll put that down there; so if I'm asked tomorrow, I'll know how much I charged them previously and 'add a bit on'.
I do a rough list of what I've done in an A5 day to a page diary as well, because it's easy to update and flick through to see what I've done the previous month.
I can also scribble notes in there to phone people for quotes and stuff too.
And I write my daily takings down in the diary too.
The spread sheet costs me nothing and takes about 5 minutes a day to update and print off the next day/weeks work list and the the diary cost about £3.00.