Strictly speaking, this identity would not be connected to SL anymore, but to metaverse.
1. First of all you need an e-mail to register on secondlife.com;
2. You have your SL account, congratulations!
3. If you sell lindens you may want to get them on your rl account, then you need a paypal account, e-mail as login;
4. If you sell SL goods, you probably need an XStreetSL.com account, here you need your SL name as login;
5. If you want to share your pics you may need Flickr.com account (Yahoo! service);
6. If you want to share you pics via BlogHud.com you need an account there too. Luckily here they ask your SL name as login, but first you need to buy a HUD;
7. If you want to blog, you may choose between Blogspot.com (Google service) and Wordpress.com;
8. If you want to socialize you may want to register on plurk.com . One account plus 
and you may plurk to Twitter, Facebook and some other networks… oh and there is a HUD to plurk from in-world too;
9. If you want to socialize but prefer Twitter.com, that is also a possibility to expand your list with accounts;
10. Don’t forget SL social networks such as Avatrait.com (art) and SecondLifeBloggers.ning.com based on Ning.com platform;
11. If you want to keep a bit of overview of all your accounts, you maybe need one extra account on MyBlogLog.com (Yahoo! service);
12. SLuniverse.com is also a comminuty platform with forum, pics, ads and more.
13. I am not finished….
….
36. Facebook.com
37. Google Talk
38. Yahoo Messanger
39. MSN
40. LinkedIn.com
….
67. issuu.com (publish platform)
68. calameo.com (publish platform)
…
98. YouTube.com
99. Last.fm
…
and on
and on
Tips:
* first choose your sl name
* make an email that mirrors the name
* register on secondlife.com
* try to keep your accounts consistent accross networks.
* too many blogs and posts to handle? check this tutorial email2blog
Questions:
a) which activity do you prefer? Doing business, to blog, sl photography, socialize, explore?
b) which platform seems better to you: yahoo or google?
c) Do you need all those accounts?