TweetLens is a web-based Twitter client. Built by Benson Wong aka @mostlygeek here in Vancouver, I find myself using it to browse my main account when I want to catch up on reading. Note that it is most definitely in alpha, and that you do need to enter your Twitter user / pass to login. I expect Ben to implement OAuth real soon :P
I use twhirl as my main desktop client, open to multiple accounts and with all notifications turned off except for @ replies and direct messages. This means that most of the time, the posts of the people that I follow just kind of scroll by in the background, and I only switch to it when I have something to post. This means that I don't actually read the majority of posts (sorry...but I *do* notice all mentions of @bmann); scrolling backwards through the twhirl interface (while new stuff is still coming in) doesn't work that well ... and, I'm really just scrolling, not really reading.
I think you could call TweetLens the Google Reader of Twitter clients. It has keyboard shortcuts for reading through and acting on posts. Like Google Reader, I can hit spacebar and "close" individual posts, visually indicating that I have actually read something. Like I said, I find this super useful when I'm taking the time to actually sit and "catch up" on reading through posts. I use twibble on my Nokia phone on the go, and just like Google Reader, it would be great if a mobile version of TweetLens saved my read / unread state.
When Ben first told me he was building a web-based Twitter client, I thought he was more than a bit crazy. But, I do find TweetLens interesting, and I'm looking forward to seeing it evolve. It's very much in alpha right now, and Ben is actively working on it, so expect some bugs or outages at times. See Google Code for the issues / features list, and follow @tweetlens for updates.</a>