As a newcomer to the internet tablet, I've learned to always look for a way to do things through the web browser.
I signed up for a callwave account a while ago because it lets you get your voicemail online (or through a handy dashboard widget), but now with my N800 I find that I can send text messages, get history of received calls (even when I don't have service), messages via "Vtxt GIST" (a service that interprets, sometimes accurately, voicemails as text), keep track of contacts, and even place calls through the browser. Unfortunately listening to messages is falls prey to the flash limitations of the device. All in all, for a free service, call wave gives tablet enthusiasts and web 2.0 afficionados a way to manage their calls in the browser.
Let me know what you guys think, and also what web 2.0 sites you make use of on your tablet!