I don't know if you are using the Office package, but there is a way of using MS Word to create a mail merge under 'Tools'.
You will first have to create a data file in either Excel or Access to store your friends Names (as you wish to see them in the letter) & E-mail addresses. Just the two 'Fields' are needed.
In your document insert the Fields and then after typing your letter click merge mail. You should see the number of entries you have made in your data file appear at the top. From this, just choose which entries you want to send a letter to from the names that appear in the, 'Name' field.
These letters should then appear individually in the Outbox of Outlook when you next open it. When sending you may be prompted to send each mail, this is a security patch in place by MS.
You will find better help in the help files of MSW.
Couldn't quite see how with mail merge even though I use it regularly for snail-mail stuff.
Did find a useful free (well for a trial period) add-on called "send personally" www.mapilab.com. Gives you a "Send Personally" icon next to "Send". Does the job exactly. The recipient sees only their name in "To" field,even though it was sent from a 73-strong distribution list. Found some other great add-ons their too.