Possibly the Single Best Piece of Brewing "Equipment" I Own

... is actually a piece of software. I personally use BeerAlchemy for the Mac although I know other tools are out there. I've briefly played with BeerTools and HopVille but I ended up ponying up for a license of BeerAlchemy for a couple of reasons. Firstly, it's done in the Mac school of design and is pretty damn intuitive. Secondly, it's easy to use along the learning curve of brewing. It doesn't make you use the more advanced options right away, so you can delve deeper into the program as you progress as a brewer.

There are a few reasons I think good software is one of the keys to actually moving from a beginning or intermediate brewer to an intermediate / advanced one.

1. Consistency: Some of my early extract batches turned out great, some were so-so, and a couple were bottle bombs. If you asked me what went right or wrong, I simply couldn't have told you. With software (or even a paper-based brew log) I can track my ingredients, my process and any miscellaneous notes along the way. So if I like a batch and want to brew it a second or third time, I can pin down which variables changed and ultimately get to the point where it's possible to make the same exact beer twice.
   
2. Process: Taking and recording readings through the brew process alongside of the process you used for a particular batch dramatically helps you understand whether what you're doing is working properly. If you've ever brewed a batch which turned out way too thick or thin for the style you were shooting for or ended up with a beer which was so sweet your friends couldn't choke it down, chances are something is off in your process. Once you start taking gravity readings at various key steps, you can discern where the problem lies.
   
3. Recipe Design: One of the best parts of actually using software is keying in a recipe and watching the resulting gravity numbers, color and IBU's change based on your ingredients. I think that simply going through this process repeatedly can make an enormous difference in one's understanding of the roles ingredients actually play in a beer.

(All that said, it's also possible that it's entirely possible to brew great beer without software and this just gets the nerdy engineer in me going...)