• Alamo - 25% disc + triple SkyMiles expires 12/31/2008 (no minimum rental)
• Budget - 20% disc + 3000 SkyMiles expires 3/30/2008 (minimum 3-day rental)
• Dollar - triple SkyMiles expires 5/15/2008 (minimum 3-day rental)
• Hertz - $30 off + up to 1000 bonus SkyMiles expires 12/31/2008 (2-day & 5-day rentals)
• Thrifty - triple SkyMiles expires 3/15/2008 (minimum 3-day rental)
• (oops, omitted this one) Thrifty - Florida rentals; quadruple Skymiles expires 12/31/2008 (minimum 2-day rental compact+)
• Avis - 500 to 1500 bonus SkyMiles (3-day & 4-day rentals)

I have 5 separate rentals that are getting me 3,000 bonus miles from Budget; 1 using the Thrifty promo; and 4 using the Alamo promo..

One note, the Avis promotion offers bigger bonus SkyMiles than the corresponding promotion for United (here) and Continental (here), but those offers are accompanied with AWD discount code K024400 for CO and K019300 for UA.