An electronic project isn't complete without supporting mechanical components. Here are some links.
General Parts and Supplies
McMaster-Carr has pipe, tubing, valves, hand tools, power tools, fuses, switches, relays, storage containers, springs, and all sorts of other stuff.
Grainger is similar to McMaster-Carr.
Production Tool Supply is a broad-range supplier of...production tools (e.g., cutters).
Small Parts, Inc. have a great selection of things like tubing, O-rings, fasteners, cable, plastics, gears, etc. Also see the tool distributors on the UsefulStuff page, as many will also sell similar supplies.
W.M. Berg makes miniature precision parts like gears, couplings, bearings, fasteners, etc.
Happ Controls has all sorts of neat pushbuttons and accessories for the "amusement, vending, gaming, home entertainment, interactive, and industrial industries".
Master Bond Inc. makes adhesives, sealants, and coatings (epoxies, silicones, cyanoacrylates, anaerobics, polyurethanes, latex systems, acrylics)
Microcut makes precision and specialized rotary cutting tools like end mills and cutters.
Onsrud sells cutter tools for wood, plastics, and composites.
End Mill Discount sells lots of end mills for fairly low prices.
Harvey Tool sells all sort of cutting tools, including specialized tools like plastic cutters, corner radius end mills, tapered end mills, etc.
SDP/SI sells all sorts of drive products like pulleys, bearings, belts, couplings, motors, etc.
Micro-Mark sells a wide variety of small tools for hobbyists.
Enclosures
TEKO Enclosures has lots of good info (including prices) online. Distribution is through [http://www.alliedelec.com Allied].
LMB Heeger has a great selection with prices. You can order direct from them or through Digi-Key.
Polycase has lots of plastic cases available, direct online ordering, good customer service, and very fast delivery.
BUD Industries is a favorite among hobbyists and is widely distributed by the major distributors.
Hammond Manufacturing Electronics Group has lots of cases/enclosures and is widely distributed by the major distributors.
PacTec makes all sorts of plastic enclosures, including specialized ones like remote-entry key fobs. You can order direct from them (though some cases have large minimum orders).
New Age Enclosures makes some standard plastic enclosures along with some specialty products such as USB "memory stick" type enclosures, handheld remote control, etc.
CaseCAD makes custom foam inserts for carrying cases.
Unibox Enclosures sells small plastic boxes.
Serpac makes a wide variety of electronic enclosures.
Box Enclosures makes a wide variety of electronic enclosures.
Device Support
Ironwood Electronics makes all sorts of sockets and adapters for SMT devices.
Emulation Technology also makes all sorts of sockets and adapaters for SMT devices.
Schmartboard makes SMT breakout boards that also allow for easy soldering of SMT parts since the parts seat themselves automatically onto their pads.
Random Stuff
Bolt Science -- everything you ever wanted to know about bolts. Trust me, there's a lot you don't know.