It's something all CG artists should have. A huge, unwieldy texture library taking up 50 gigs of your hard drive space. I'd commit the whole thing to a keychain thumbdrive. You never know when you are going to need to texture on the fly.
Organization is key! I'd suggest some essential top-level categories: stone, metal, wood, organics, fabric, terrain, fx. Depending on your professional focus, others may be appropriate (scifi, glass, ceramics, plastic, foliage, grunge). Some with two or four-way tiling, some for rubberstamping information from as you build unique textures, some just for reference as you create stylized or hand-painted textures; some with vignetted edges for use as alphas or brush heads. It's also good to have a naming convention or subfolders indicating appropriate usage. "Tilable" to indicate tiling textures; "decal" to indicate a unique feature like a rivet or latch; "base" to indicate a background from which unique textures can be built-up (for example, a close-up of fabric showing no unique folds or shadows, but fairly even in tone throughout); "dirt' or "grunge" to indicate an image from which info for a grunge layer may be sampled.
In addition to always carrying a cheap digital camera with you (with the flash turned off!) you can dumpster-dive the web for a lot of free textures to get your collection started. Here's a smattering of links to help. Happy collecting!