Golden Cheetah: Takes a bit of getting used to, but really excellent IMO. If I were a cyclist I'd use this exclusively... it has everything that training peaks offers (PMC etc) and more (better CP chart, workout comparison tool, aerolab etc), however it is worse than rubbish when it comes to swim and run data (it refuses to acknowledge swimming even exists) and so it isn't really an option for triathlon.
Training Peaks (premium): Good, but the cycling metrics just aren't as good as in GC. For instance you can't view sessions side by side by side to compare different TT efforts, and the CP chart only gives you data for 3hr, 90min, 60min 30min, 20min etc. whereas GC has time set as a continuous variable so you can view your CP for 23:45 or whatever you want. The only chart that TP has that I miss in GC is the "power profile chart" although you could approximate it by creating a weekly CP5/20/60 chart in GC. I only use Training Peaks for 2 reasons... 1 is to connect with my coach, and the 2nd is that it handles swim and run data well, meaning that my PMC in TP is roughly accurate.
Strava: Facebook for sport. Nice user interface but not useful for any serious analysis, but it is useful for stealing other peoples data if you want to know how someone beat you in a TT, or for comparing your progress over particular segments (in my case the climbs I use for hill reps etc.), or for finding new routes.
WKO: I hope it will fill the gap between GC and TP, however Ian has had problems with it* and I just paid for another year of TP premium so I won't be trying it out any time soon.
*btw Ian, did alex simmons response help you out at all?