Here's a slightly more succinct version inspired by code from this article http://www.simple-talk.com/sql/t-sql-programming/concatenating-row-values-in-transact-sql/

SELECT t1.col1,

(

SELECT

CASE WHEN ROW_NUMBER() OVER(ORDER BY col1, col2)=1 THEN '' ELSE ',' END + col2

FROM

TestTable

WHERE

col1 = t1.col1

ORDER BY

col1 ,col2

FOR XML PATH('')

)

FROM

TestTable t1

GROUP BY

col1