http://www.livecard.net/
Create cusom cards like this to show off your profile a bit differently.
http://www.xbox360achievements.org/
Detailed achievements list with how-to info.
http://www.360voice.com/blog.asp?tag=CMorrigu
Let your 360 log your game playing (available as http://syndicated.livejournal.com/cmorrigu_360 )
http://profile.mygamercard.net/CMorrigu
So far, I like this achievement list the best. It has direct URLs so you can bookmark/show off the list, and plenty of stats.
http://www.xboxusersgroup.com/forums/vbgamerboard.php
Nice achievement details list, can't find direct link to mine, just use the search box.
http://www.top360tag.com/
Some achievement details, again no direct link that I can find.
http://www.gamertag.com/
Sign up for some evident benefits, not sure yet.
http://www.seatag.com/
Blogs, stats, etc.
http://www.xbox.com/en-US/community/developer/showcase.htm
See new online toys here.
http://www.gamertagpics.com/
See (supposed) RL pics of gamers.
http://www.majornelson.com/
News, MarketPlace announcements (available as http://syndicated.livejournal.com/xbxmarketplace/ )
That's about all I've found so far.
I bought 4 Xbox Live Arcade games so far, and I grabbed Fable and Lego Star Wars for the Xbox1 the other day. I even ordered the Vision camera pack, which includes 12 month Live Gold, 3 games, and 200 points. That should be in this week sometime.