"Life's too short to use cheap tools."
For basic hand tools I think Sears, Depot or Lowes are great. They have high quality tools, made right here in the good ol' USA. They rely on volume so don't expect to find unusual or specialized tools.
For a DIY'er it might be hard to justify Proto or Snap-On on purely practical grounds but there's just something about working with a really nice tool that feels right. I've never regretted saving up a little longer and spending a little more on a top quality tool. I've almost always regretted buying a cheap one.
I've pretty much got enough Craftsman tools so now I mostly find myself buying from industrial suppliers. It helps to live in the world's foremost metropolitan/industrial/aerospace neighborhood but these days the world of tools is a mere mouse click away. Everything's as local as the guy (or gal) in brown shorts.
Chasing down the local Snap-On, Matco or MAC tool truck might feel strange at first but I highly recommend everybody try it at least once. (Start with something small to avoid excessive sticker shock) You just might get hooked.
Amazon has good prices on power tools. I always check them first. They stock most of my favorites, Milwaukee, Bosch, Porter-Cable, Fein and lots more. For Festo you have to go directly to the Festool USA. No problem, they have excellent customer service. There aren't too many places to get Metabo but with a little googling they'll turn up.
For specialized tools I like to stick with specialty suppliers. There lots of online suppliers for electronic tools like Weller, Klein, Xcelite or Erem. I Usually get measurement tools, Mitutoyo, Starrett, Brown and Sharp, from Enco or IPS.
My favorite source for detailing tools and supplies is Top of the Line in Arkansas but I usually get chemicals locally from Detailing Depot in Costa Mesa.
For woodworking tools I usually check with Rockler, Woodworker's Supply,Klingspor or Lee Valley first but there are tons of good suppliers out there. I'm still trying to convince myself I need some traditional Japanese hand tools.
I buy bicycle tools from my favorite local shop, Two Wheeler Dealer in Brea, CA. Mostly I'll be rational and buy Park or Wrench Force but sometimes you absolutely have to get Campagnolo.
I buy ski tools from Tognar Toolworks.
PC.