My favorite tools are the simple ones. An old wood mallet, a scribe, some 1-2-3 machinist blocks, a steel ruler on the edge of my bench.
Every day I'm in the garage, I use this thread gauge. It's got most of the common nut and bolt sizes in imperial and metric. It's always nice to do a quick check to see if you need fine or coarse threads to match up to the bolt you're using. Saves a lot of looking!
