Depends on what you're looking for really in terms of size and power.
The first one I built was from http://mikebeck.org/
taught me loads about how a CNC works but wouldn't be a step up from your Shark.
I bought the plans for http://joescnc.com/
but never got around to building it. Great machine, great forum and lots of Vectric users. Probably would be my number one choice now it's rack and pinion as well.
The other machine I went a long way down the route of getting was from http://www.mechmate.com/
serious piece of kit but I would have had to lay a concrete floor in my workshop to support it. Again a well supported forum.