Firstly, almost any loose fitting top pattern can be used, and, depending on the style, it may need a little adaptation.
This, below, is example of a non-maternity pattern which can work as a maternity top with just a small adjustment.
I used this pattern to make this nice animal print top for my pregnant daughter. It's loose right now but I'm hoping this will take her through the next three or four months.
1) Obviously, you can just add more width by flaring out the sides on the front.
2) With a fairly straight top, you can also lengthen as well as widen the front, and elasticate the front sides into the side seam. I've seen examples of this done but when I tried, I found it sagged down too much at the front (certainly while she is only half-way through) so I took this elastication / gathering of the side seams out.
3) For this animal print one, I adapted the pattern by adding a centre pleat. The way I did this was to add a few inches more to the width of the centre front pattern. (About 4", as I was using a soft jersey knit)
Then, I folded the material of the centre front right sides together, made a short seam down from the neckline where the centre front of the original pattern was, down to just where the bust started.
I then opened out the two sides, and pressed the 'revised fold line with the centre of it against the seam I had just made, so that there was equal material either side of the seam. This made two side folds to the pleat.
Then I sewed the neckline of the pleat to the original neckline, The effect of this was to form an inverted pleat down the centre. You can perhaps see how that is working as the waistline is expanding.
This top I made with the sleeves as in 'D' in the pattern - that's the blue patterned one with the solid blue hem.
However, since I first wrote this post, I've now found an interesting article on the same subject from Melissa at Melly Sews, whose patterns and tutorials I very much admire. She has an alternative approach (or approaches) to pattern adaptation which would work especially well for patterns without an obvious easy way to make them fit the maternity bump. You can find her excellent article here.