The City of Austin actually runs a day labor service. It is located on the Southbound I-35 frontage road between 51st and Airport. Always plenty of people there, and you can even request someone online (
http://austintexas.gov/daylabor). Also, you pay the workers cash directly, so no 'management' fee that private companies charge.
I've used them for everything from moving homes, painting, landscaping, and even a ceramic tile walkway.
As for pay, it is generally $8-10/hr for non-skilled like manual labor, and $10-15/hr for skilled labor such as welding/electrical/plumbing etc...
With the heat we're having, I'd pay them $15/hr and include an ice cold Gatorade.