Some 'old' favorites that I have used for months are on this list of Web2.0 award-winning sites.
I StumbleUpon videos from YouTube and MetaCafe all the time, regularly refer to Kayak to find travel deals, use Facebook to play Scrabulous with my librarian buddy in Australia daily and on occasion to check in with another group & share their photos.
First experiences probably have a lot to do with how you do Web 2.0 - since I did Picasa before I saw Flickr, I think Picasa is easier; I would place StumbleUpon (especially with the new bookmark integration with Firefox3 -wow!) before delicio.us, Pandora music project before Last, GoogleBlogsearch way before Technorati.
I do soooo love my GoogleMaps, PBwiki, and now GoogleReader! Couldn't live without any of them.
Omnidrive to synchronize files (free) may be what I am looking for to keep things flowing between home and school computers... IF it can be accessed at school! We'll see in August.
My students may be able to share sizeable documents through docstoc, if they mark as private and give private url to one another; worth looking into later, since sometimes large ppts and other image-rich docs don't go through e-mail well.
Myheritage.com looks like a good alternative to the $$ genealogy sites out there - but I will be playing with it more when I have plenty of time to spend entering names,,, looks addictive!
Zango - fairly standard gamesite; Galaxiki.org - whoa, buy your own star and create your own galaxy!? hmmm... No Doof for me - I don't really want the social aspect of gaming to be so open (I want to know who I am playing against, old fashioned me) and prefer single-player games anyway.