One of the most popular bulbs there is, is GU10 bulb. Often used for downlights and spotlights it has a built-in pin pitch, with the two pins spaced 10 mm apart from each other. In the past this bulb was only available with halogen technology but since the LED technology surfaced there is an endless choice of GU10 LED's on the market. GU10 bulbs come in different temperatures and colours.
When it comes to retrofitting GU10's, in most cases they can be swapped quickly and easy for the new energy saving options.