Twitter is all the rage. It was name-checked on Conan's second Tonight Show broadcast and was even extended into a comedy bit. Basically, Twitter is a microblog that you post, which is shared with the world at large. You can collect friends and only see their tweets, see everyone's tweets, and even reply and send direct messages. In order to use Twitter, normally you have to stick to its Web site, twitter.com. Hence the need for a standalone PC application, and freebie Spaz is one such approach.
Unfortunately, Spaz is notable more for what you can't do than what you can. An app should at least have all the functionality of the Web site itself, and in that respect Spaz falls short. You can't follow a conversation, making posts seem disjointed. You can only see a list of a user's most recent tweets if you click on the user's name and open up said timeline in a browser window, which seems to defeat the purpose. The point of a standalone tweet app is to avoid having to go to the Twitter website whenever possible.
Unlike with chief competitor TweetDeck, you can't have multiple columns with your timeline, replies, direct messages, and so on, all on one screen. You have to toggle between them, which isn't as useful.
All in all, Spaz is free and light and it's open-source (the code is available to be user-modified), so those are two things it has going for it. However, it can't approach other Twitter apps like TweetDeck (which is also free) in terms of versatility and customizability. And it requires Adobe Air to be installed already, which adds a layer of hassle (especially if this is the only Air app you'll be using). You're better off looking elsewhere.