This is not a typical post, but I just had to share as it took me a good afternoon of trial error to figure out how to add a new contact to a Constant Contact account using their REST API. Their documentation has many holes, but fortunately the API works well.
After you've authenticated, here's the XML required to add a new subscriber:
<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
<link href="/ws/customers/username/contacts" rel="edit"></link>
<id>http://api.constantcontact.com:443/ws/customers/username/contacts</id>
<title type="text">Contact: email@address.com</title>
<updated>2011-02-27T18:48:00.760Z</updated>
<author>
<name>Constant Contact</name>
</author>
<content type="application/vnd.ctct+xml">
<Contact xmlns="http://ws.constantcontact.com/ns/1.0/" id="http://api.constantcontact.com:443/ws/customers/username/contacts">
<Status>Active</Status>
<EmailAddress>email@address.com</EmailAddress>
<OptInSource>ACTION_BY_CUSTOMER</OptInSource>
<EmailType>HTML</EmailType>
<Name>this doesn't seem to work - use firstname & lastname instead</Name>
<FirstName></FirstName>
<MiddleName></MiddleName>
<LastName></LastName>
<JobTitle></JobTitle>
<CompanyName></CompanyName>
<HomePhone></HomePhone>
<WorkPhone></WorkPhone>
<Addr1></Addr1>
<Addr2></Addr2>
<Addr3></Addr3>
<City></City>
<StateCode></StateCode>
<StateName></StateName>
<CountryCode></CountryCode>
<CountryName></CountryName>
<PostalCode></PostalCode>
<SubPostalCode></SubPostalCode>
<Note></Note>
<CustomField1></CustomField1>
<CustomField2></CustomField2>
<CustomField3></CustomField3>
<CustomField4></CustomField4>
<CustomField5></CustomField5>
<CustomField6></CustomField6>
<CustomField7></CustomField7>
<CustomField8></CustomField8>
<CustomField9></CustomField9>
<CustomField10></CustomField10>
<CustomField11></CustomField11>
<CustomField12></CustomField12>
<CustomField13></CustomField13>
<CustomField14></CustomField14>
<CustomField15></CustomField15>
<ContactLists>
<ContactList id="http://api.constantcontact.com:443/ws/customers/username/lists/2">
<link xmlns="http://www.w3.org/2005/Atom" href="/ws/customers/username/lists/2" rel="self"></link>
<OptInSource>ACTION_BY_CONTACT</OptInSource>
<OptInTime>2011-02-27T18:48:00.810Z</OptInTime>
</ContactList>
</ContactLists>
<Confirmed>true</Confirmed>
<InsertTime>2011-02-27T18:48:00.760Z</InsertTime>
<LastUpdateTime>2011-02-27T18:48:00.760Z</LastUpdateTime>
</Contact>
</content>
<source>
<id>http://api.constantcontact.com:443/ws/customers/username/contacts</id>
<title type="text">Contacts for Customer: username</title>
<link href="contacts"></link>
<link href="contacts" rel="self"></link>
<author>
<name>username</name>
</author>
<updated>2011-06-03T17:14:10.175Z</updated>
</source>
</entry>