Firstly there are market research firms which specialize in doing this kind of research, it tends to be expensive but if you have a local business library you m

Firstly there are market research firms which specialize in doing this kind of research, it tends to be expensive but if you have a local business library you may be able to use it to get free access. For your J2EE example I'd have a look at the existing products out there and see how many users they have (if it's open source look at their download figures, if it's commercial see if the company accounts are publicly available). Remember you don't need a precise figure, an order of magnitude will do for judging a market size. For your web based staff planning application, I think you need to be much more specific about what your product does. If you can't figure out which business your product will apply to then you'll have a hard time selling to them. Start by thinking how you'll identify customer to sell to and the generalize from that.