microsoft excel, open office, or google docs spreadsheet
columns labeled for each detail you want to record, and each row records the date and details that are recorded on that date, start a new sheet for each cycle.
The details which should be recorded will vary depending on how you are growing. Just think about each thing you check when you go through your grow and decide if it is significant enough to record. I have a column to record everything I check and an additional generic column for "noteworthy" where I normally record my activities such as taking clones or transplanting.
ive been using this for a couple of yrs and keeping track of 8+ strain prepetual grow would be alot more difficult. it will warn you when its time to flush & harvest and you can document each harvest with notes and pics.