I usually recommend Dell.
Check GotApex.com, they have deal listings all the time (not just Dell). Sometimes Dell sells their leased equipment (I forget the name, Dell Business Services, I think?), and sometimes you can get REALLY killer deals there. But sometimes, the prices are worse than buying new, haha, so you kind of have to wait for the "sales".
I was really, really tempted a while back, they had a really great laptop, about 1-2 years old, for like $250. I ended up not getting it, just because I didn't want to spend the money. But I did get my current computer through them, it's a fantastic computer which, based on the specs, would have been like $1000, but they had a sale AND a coupon (gotta love double-dip deals), so I got it for like $300.