If you want to lift heavy things to get strong, then you need the heavy things. If you are building and using different weights for different exercises, along with a mix of bars and barbells, you either need to invest in significant space requirements and equipment volume at home, or go to a gym.
I am no expert in the different types of strength training, except that there is a large variety and I have tried a number of different options myself. I have found significant benefits though from a home strength workout that can be done with very little equipment. This workout is geared towards runners, and while it may not lead to full-body bulk, I definitely notice a different in my overall strength when I do it regularly.
Many triathlon books offer other exercises and workouts that can often be done with items found at home, and which target good base muscles for triathlon. Some also find yoga to be a good options, and there are some great videos that will bring yoga workouts to your living room for a fee or free of charge. I wrote about my favorite here.
Obviously there are a lot of considerations for a gym vs. at home, but if you can get yourself to do the workouts from home it can save money as well as commuting time! Tomorrow I will wrap up this series with an overview and some of my own observations regarding money and our choices.