The Adafruit PiTFT screens are nice screens, and well-supported, but note that there are, I believe, nine
different models under that name at this point (it's more of a family than a single device). They go something like this:
If I'm looking at the picture on eBay right (and if they borrowed the right picture), that's a PiTFT Plus #2298
, with the 40-pin connector (for newer Pi's), and the resistive touchscreen and on/off backlight. You might be interested in their #2423
, which is basically the same, but with a much nicer capacitive touchscreen and variable backlight.
Pretty much all the small 3.5"-and-under screens for the Pi connect to the Pi's GPIO connector and use SPI to get data to the screen. And they all
need Raspberry-Pi-specific drivers in order to work. One can't hook up just any SPI-based screen and have it work, but there are many screens where either the manufacturer, or some enterprising individual, has done the work to get it supported on the Pi. A while back, a collection of drivers, Notro's Linux Framebuffer drivers for small TFT LCD display modules
got added to the standard release. It used to take a specially compiled kernel to get screens like the PiTFT running, now it can be some config file tweaks (though there's still a lot of older documentation floating around that insists you need a special kernel). Anyway, it's important to find out what's necessary to get any particular display up and running, before buying. Screens from Adafruit and Waveshare
are popular choices. Keep in mind that these are framebuffer screens getting sent data over SPI from the CPU. They are not especially fast.
As W. H. Heydt mentioned, the "Official 7" Raspberry Pi Touchscreen" is also quite good (and the only one that can plug directly into the Pi's DSI connector, where it gets the full effect of the GPU). The other alternative is small HDMI monitors - I've seen a few posted around here that fit on top of the Pi like these smaller screens, with a sort of U-shaped HDMI connector to plug the board/screen into the Pi; being HDMI-based they also get the full benefit of the GPU.