D90 has certain pro-oriented features that may not be relevant to your needs. D90 is older model prosumer model that has been replaced by D7000, the current top dog in Nikon's prosumer line up. I'm a Nikon user; therefore, I may be biased but all of the above mentioned cameras are capable, feature-laden models that should fill your needs for the foreseeable future unless you decide to turn pro.
D3100 is probably most beginner friendly model with all sorts of scene modes and in-camera tutorials. Despite being a beginner oriented DSLR, it is quite capable and more advanced than most point and shoot cameras. It has been replaced by D3200, with more advanced sensor and a totally new feature that will start to show up more and more in digital cameras - WIFI adapter that connects to the camera and allow you to transmit images to your smartphone and other devices. For some, that feature alone makes it a strong candidate for purchase over other comparable models. D5100 is bit more advanced than D3100 and has a better sensor for better overall IQ and few more features, although its sensor may not be good as newer D3200.
Canon T3i is exactly same as T2i, except it has articulating screen which is useful for video purposes. Innards are identical. T2i is usually $50 - $100 cheaper than T3i. I would suggest T2i over T3i unless the articulating screen is important to you.
I recommend that you go to a local Costco, Best Buy or Sams and see how these cameras feel in your hand and see how the menus and buttons are laid out.
D3200 is currently priced at $699 with capable 18-55mm kit lens that performs better than Canon's version of that lens.
