SELECT [TableName] = so.name, [RowCount] = MAX(si.rows) FROM sysobjects so, sysindexes si WHERE so.xtype = 'U' AND si.id = OBJECT_ID(so.name) GROUP BY so.name ORDER BY 2 DESC
The sysindexes table is usually a little bit inaccurate, because it is not updated constantly. It will also include the 'dtproperties' table, which is one of those hybrid tables that falls neither under the 'system' nor 'user' category. It does not appear in Enterprise Manager's "Tables" view if you choose to hide system objects, but it shows up above.
In any case, it is generally not recommended to query against the system objects directly, so please only use the above for rough, ad-hoc guesstimates.
This post is from my old blog posted on February, 2011, that unfortunately no longer exist.