Practical Machinist, similar to Home Shop Machinist but more oriented towards those who do metalwork as a living. I prefer Practical Machinist, but the stuff described is certainly more industry oriented. The South Bend forum is like a mini Home Shop Machinist, and a great resource for people new to lathe work: http://practicalmachinist.com
McMaster-Carr, the first place I go when sourcing oddball parts for an upcoming project. Whenever I do a design, McMaster is a tab open with various fasteners or springs up on the screen: http://www.mcmaster.com
Enco, industrial supply. Cheaper (and better, in the sense that they tell you the who and where of the product's manufacture) than McMaster for most tooling for the shop, but smaller selection. Cheaper than MSC and Rutland, but basically the same products. Lots of free shipping and 10-20% off offers, just google for the codes. http://use-enco.com
Online Metals, a great place to get small quantity metals of some specific size. Comparable prices to McMaster, but more reliable in providing material certs (can be important, such as on the rifle project). Also a slightly better selection in some areas (they sell maraging steel!). Still, better to get general stock at scrapyards to save money, if you can: http://onlinemetals.com
Corbin's Treehouse, a friend I met through unicycling. He recently started a shop, and has had some cool projects, including a tandem unicycle. Definitely worth checking out: http://www.corbinstreehouse.com/