There are a number of sites that I use like applications on a daily basis. Listed below are some most known and maybe, just maybe, a few that aren’t.
flickr – this is the picture mecca. If you upgrade you can download everything you every take with a digital camera. And the best part is the Contacts and Groups areas that you manage. Upgrading to the Pro account is well worth it.
Twitter – the Mac has a neat program Twitterific which makes twittering easy. When you get used to it, and you have to because you will initially discount it as a useless program, you can see what it is all about especially if you attend a conference with others of like minds.
Facebook – not much to say here as the main media has been pounding out information. You need to manage exactly what you want to use this interface for. One of the things that to me is amazing is the ability to quickly send a video on the wall. You can save a ton of time videoing instead of writing.
Jott – this is something I use all the time. Walking, driving or whenever you don’t have a pen or paper, send yourself a quick note (usually via cell phone). Sure you can get the audio message but it also is transcribed into text where it can be forwarded into an email or text message.
YouTube – who doesn’t know this one but again they have the facility for you to record and send a video to a specific person. Some may not have a Facebook account.
Linkedin – professional networking. It goes beyond Facebook in that it has a business focus.
tinyurl – change those long and cryptic URLs into something manageable. Simple and effective.
Simpy – the social bookmarking service. It is simple, works great and keeps bookmarks in one space. It has features galore.
WordPress – the most enjoyable blogging site. The more you spend time with this the better you like it. And if you can host WordPress on your own site it will take you days to get through all the themes and widgets offered.
I am sure some sites have been missed. But think about this – would you have thought any of this was possible 5 or 10 years ago?
Tinfoiling 
All of these are good. And I use all of them, but the one you didnt mention (probably because its not quite a service like these) is iGoogle or some other rss aggregation site like NetVibes or Pipes.
I have like 9 tabs of compartmentalized resources on iGoogle. That is how I “internet” 🙂