I finally got back to reading some of my “Introducing Microsoft SQL Server 2005 For Developers” book and I stumbled upon the topic of user defined aggregates (UDA). I think this is an interesting subject, although probably only useful in very distinct scenarios.
The book talks about using it to concatenate a list of strings (e.g. e-mail addresses). While this works after some modification of the code in the book (due to beta differences) it really isn't perfect for this situation, because the maximum size of the serialized UDA is 8000 bytes. (I'm not sure why this value was chosen as the maximum size.)
The following article provides information on creating the UDAs: http://geekswithblogs.net/tonyt/articles/70274.aspx.