I often use SeaTacPark.... if you grab the coupons out the AS in-flight magazines (although not as good as they used to be), plus take into account you get 5 free days parking for every 25 (equivalent to 1 free day for every 5 paid), it ends up being a good deal. Excellent service usually. Park your car and they pick you up immediately. Use their website to guarantee your rate. I liked the old punch card system, as the agents would often sneak a few extra punches in at times... now its all automated and legit. (Darn! hehehe!)

ShuttlePark2 is also great. Cheaper initial rate, but comparable to SeaTacPark over the long run, and a little close to the airport. I used to dislike their service (slow picking you up in the lot), but they've changed to operate more like SeaTacPark, so I use them occasionally too. User their website for their web rate.

Another little tibit of parking info... you can now park at the airport for $14.95/day ALL IN... flat rate. Business travelers can now sign up for a frequent parking card that has this $14.95 rate. This is very comparable to MasterPark Lot A, especially when you add in MasterPark's taxes and fees. The Sea Tac $14.95 rate includes taxes and fees. I use this when I'm only gone for a day or two, or if I'm running so late, I can't take the chance parking off site. Oh, and you also get 10% off Starbucks and SBC coffee in the airport.