This company has an amazing search engine. I often use it to select parts while I am working on a design. They keep millions of different parts in stock. They are located in the U.S. but they ship globally and prices are viewable in different currencies. If you are looking for an item they carry, there's a very good chance they have it in stock. If an item is out of stock, you can see the expected lead time until they arrive. They usually ship within an hour of placing the order. They have a very good sales and tech support team. The only negative is that their prices tend to be higher.
Allied does business in the U.S. and Canada only. They have a relatively poor search engine. They carry products from a couple hundred manufacturers. They seem to be downsizing recently. Pricing is very good, but many products are not in stock. They usually ship same day.
Newark is now part of the Premier Farnell group. Their search engine is pretty good. They carry products form over 400 suppliers. Pricing is good; most items are in stock. Sales team has flexibility to adjust pricing. Same day shipping in most cases
Avnet Electronics Marketing:
Their search engine is user-friendly and very powerful. They handle around 160 product lines. Their pricing is often a little high. They have a very generous samples program, and offer quite a few training opportunities, many of them free.
They have a very good search engine. They carry a lot of parts, but they tend to be low or out of many of them. Pricing is very good, but they do not carry Microchip. Arrow's home office is in North America. They have a presence in the Asia Pacific, Europe, Middle East, and Africa. I only have experience with their North American Components (NAC) division. They usually ship same day.
I am somewhat hesitant to add Jameco to the list of heavy hitters. They have historically catered to the hobbyist, but in the last year they are going after OEM's too. Pricing for name brands is usually a bit high. Be cautious when you search for a part, especially passives, because they will often match the part number you entered with a "house brand" Chinese substitute. I have used their house brands with mixed results. Tantalum capacitors were fine; power inductors were undersized and ran hot. Some of their stock items are "one-time-buys".