2.54mm (0.1") pitch sockets, available in 3 or 4 way.
Why? Use the 3-way if you fancy experimenting with different transistors in a circuit, or if you're just not confident about soldering them
4-way are very handy if you're building kits using the 3PDT Direct Connect Ribbon system. Instead of attaching wires to your bare ribbon cable ends, which you'd then have to desolder and clean before reattaching to your daughterboard, you can solder four wires to one of these sockets. Then each time you're testing a circuit you can just pop the socket onto your ribbon cable. Bam! No mess, no desoldering.
There are places charging 99p a go for these on ebay. FFS!