The answer may depend a bit on why you want the smaller form factor.
If it's just being a bit cramped on floor space, the best answer is to make some more floor space and get a Bridgeport-type. Even a couple thousand spend to bump out a wall or clear something out will likely save more than trying to get a smaller and less capable alternative. Even that Bridgeport is a toy in the minds of some; but it's hard to beat for all-around use.
If that extra floor space proves difficult or expensive to get, then the options include a Bridgeport with a short table, a Millrite, Clausing, Rockwell, and various import 2/3rds size machines. I've seen sturdy Taiwanese vertical/horizontal mills (maybe 30 taper) with about the same footprint as a Millrite and their owners seemed to like them.
If you don't need much of a work envelope then the small sized entries in the Deckel, Maho, Aciera . . . forum would seem ideal.
I've owned and used several of the small alternatives suggested above and can comment. The Versa-Mill is a very cool complement to a lathe. It's great for milling spur gears, small details on a shaft, etc. The attachments are good for internal and external grinding, limited spline shaping, etc. Detached it can be moved to massive equipment and do something like mill a keyway. But it's not all that good a general purpose mill. Even a small round column mill drill might be better for squaring blocks, clearing pockets, drilling and tapping, etc.
The larger square column mill drill has close to the envelope of the shortest table Bridgeport and can do semi-serious work for a hobbyist. Its form factor make sense when it's not just square feet but cubic feet of space that are in short supply; since something like a Lista cabinet can fit under it with proper bracing added for the mill. They're not especially accurate and not designed to last in any sort of production. But at 800# or so easy to move.
The Hardinge TM and UM horizontal mills (often equipped with a vertical head) are sturdy little guys and can be pushed a bit harder than, say, a Clausing 2/3rd size mill. These are only around 800# but cut like they're a bit heavier. However, they have limited travels and are probably best suited to horizontal work. One might be a nice complement to a small vertical mill, if you can find two cubbyholes. There are at least a few folks on this forum, though, who've gotten comfortable enough with these to use them for most everything.
