Edit: I see Seesmic mentioned as favored but the description sounds a lot different than what they describe their client as doing (that is, combining tweets with other alerts, something one might imagine would stand in the way of twitter's precious user experience).
Same thing with Trillian, which I use. I won't be using a standalone Twitter client for windows, so I guess it just means I won't be using Twitter if they remove the API access for them.
http://seesmic.com/
Edit: I see Seesmic mentioned as favored but the description sounds a lot different than what they describe their client as doing (that is, combining tweets with other alerts, something one might imagine would stand in the way of twitter's precious user experience).