The Makita RT0701C trim router that ships with the Beta 01 includes a 1/4" collet, which will hold standard 1/4" shank endmills.
Despite the product description saying it does not include an endmill, my Beta 01 included a QTC (OSG) 402-2500 1/4" square-end carbide 2-flute endmill (shipped Dec 2014). I'm not sure if this will be the case for all units shipped, though. This bit seems to be a good quality bit and works well.
Additional endmills can be purchased from a variety of places, including:
Other popular home CNC machines such as the Shapeoko and Shapeoko 2 were designed around small rotary tools such as the dremel, which use 1/8" shank bits. As a result, there are a lot of endmill options in the 1/8" shank size on the market.
Makita does not offer a 1/8" collet option, although a popular choice discovered by the Shapeoko community is:
I purchased this collet and it works great. Elaire Corp also offers other collet sizes, although the 1/8" along with the included 1/4" are probably the most useful sizes for most home CNC work.
To use with this new 1/8" collet, I purchased the following starter pack from Inventables which I have found useful:
There is a lot that goes into selecting an endmill. Beyond just their shank size, there is the number of flutes, upcut vs. downcut, different end types such as ball end or v-cutting, etc. The following link is a good place to start to dive deeper into the topic: