How can I combine multiple rows into a comma-delimited list in SQL Server 2005?

SELECT X, Y FROM POINTS

X    Y
----------
12   3
15   2
18   12
20   29

XYLIST
----------
12,3,15,2,18,12,20,29

DECLARE @XYList varchar(MAX)
SET @XYList = ''

SELECT @XYList = @XYList + CONVERT(varchar, X) + ',' + CONVERT(varchar, Y) + ','
FROM POINTS

-- Remove last comma
SELECT LEFT(@XYList, LEN(@XYList) - 1)

SELECT STUFF((SELECT ',' + X + ',' + Y FROM Points FOR XML PATH('')), 1, 1, '') AS XYList

DECLARE @XYList AS varchar(MAX) -- Leave as NULL

SELECT @XYList = COALESCE(@XYList + ',', '') + CONVERT(varchar, X) + ',' + CONVERT(varchar, Y)
FROM POINTS

DECLARE @s VarChar(8000)
SET @s = ''

SELECT @s = @s + ',' + CAST(X AS VarChar) + ',' + CAST(Y AS VarChar)
FROM POINTS

SELECT @s