In my experience National Semiconductor take forever, if they even bother sending them at all, but i have had some free samples of their top quality op-amps which made my ARX compressor sound a whole lot better.
Not had any experience with TI but Microchip send samples of PIC's etc pretty quickly. I've never been able to actually get any samples from Atmel though for some reason.
If you can prove you actually run some kind of business you can get all kinds of free samples. I got a couple of plastic single unit rackmount enclosures free the other day just by inquiring how much it'd cost to buy 20.
