UMPCPortal is a great resource. The two things lacking from their product compilation is whether the product is currently being produced (e.g. the N810 shows as "available" which is technically true but "no longer in production" would be more useful) and the estimated price (which would, admittedly, be difficult since they cover devices from just past the rumor stage through end of life).

Looking for 4 to 5 inch screens, under 300 grams, with a keyboard and Wi-Fi, there are three devices - the N810, the 2007 era Sharp EM One, and the announced but not yet shipping (that I can tell) Samsung Mondi. You might take a look at the reviews for the latter.