I have Twitterific on my MacBook, and use TwitterFon on my iPhone. But I hardly turn on Twitterific though, I seem to just prefer to go to Twitter when I'm on my laptop.
I've been loving TweetDeck when I'm on both my MacBook and my Mini. I can stay away from the web browser, which gets me easily distracted from work, lol! So does Twitter but at least I'm minimizing distractions!
I'll have to check out Digsby. Right now, I'm just using TweetDeck and for the post part I like it. It was working great and then all of a sudden stopped notifying me of replies.
I use Ping.FM to send messages to Twitter, FriendFeed, and Facebook at the same time. They also have a ton of other services that they work with. I tried downloading Digsby but for some reason it just won't load right for me. That's probably a good thing. LOL
The Twitter apps I've used on my MacBook/iMac are TweetDeck, Twhirl, and Twitterific. Lately, I've started using TweetDeck almost exclusively since I like the multi-column interface. On my iPod Touch, I have tried Twitterific, Twittelator, and Tweetie. Of the three, I prefer Tweetie best.