Let me just get this out of the way: The Palm App Catalog sucks. Give it time and there will be plenty to choose from, but right now there are only a few gems in there. I was at first eager and hopeful when I saw Classic, by MotionApps, which is an emulator and allows you to install any of the applications that were designed for Palm OS on webOS. It would make thousands of applications availble to Pre owners, but two things are keeping me from purchasing right now: the $30 price tag and the plethora of poor reviews. It's too much for an app that sounds like it's basically a buggy beta.
My favorite app right now is Tweed, the Twitter client by Pivotal Labs. You can track multiple accounts, and change the font size (choosing the small font also eliminates the profile pictures). There are shortcuts for Trending Topics, Favorites, Direct Messages, My Tweets, mentions, Bookmarks, and all Public tweets. Clicking on a hashtag will open the search window. I've never had any problems with speed or crashing.
My complaint is there is no option to view in landscape. Granted, you can't actually compose a tweet in landscape mode because there's no on screen keyboard, but it would make reading tweets easier.