Are you looking for free, or a pay service? Because, some of the pay services are outstanding and offer more than just logging your training.
I personally follow this formula, and it has served me very well for the past 3 years:
1) Figure out your A, and B priority race dates and use them as a target for peaking your training.
2) Find a plan, either through a book, another group member, online resource, etc. that suits your available training hours and volume and takes into account your experience and fitness level.
3) Work backwards from the race dates the number of weeks your training plan is for in order to figure out your starting date for official training (ex: IM Wisconsin is Sept 12, 2010 - I follow a 30 week plan - starting date for official training is February 15th). If you have multiple races scheduled you will have to adjust to peak 2-3 times in a season (no more than 3-4 peak races per season at most)
4) Make an Excel spreadsheet that shows what you have to do every day, including rest and recovery days, based on the plan you've selected.
5) FOLLOW THE PLAN EXACTLY AS WRITTEN WHENEVER POSSIBLE.
6) With #5 in mind, allow yourself the occasional "off" day that isn't scheduled when you are either extremely tired from the week's training stress load, or if you get 10 minutes into a workout and just don't "have it" that day. It is your body telling you to rest for the day instead of trying to force the fitness. Your workout would be sub-par anyway and you would be better served with a bit more recovery instead of a half-assed workout.
7) If you have a Garmin or other heart rate, power, GPS device - review the log from the device the same day you workout and make notes either in the spreadsheet or in a book that notes particularly useful information about the workout. For example, temperature, humidity, wind, etc. How do you rate your performance (1-10), was it too easy, was your HR too high, did you spike your powermeter, did you have a breakthrough workout, how was hydration, nutrition, your diet for the previous day or two, etc. Things that will help you look for good and bad trends that affect your workout.
If you do all this, you will be able to look back and see some startling things about every facet of your triathlon life. I don't use an online tracking system, but have found that what I've outlined above is what works for me.
J