Elliptical training is an excellent form of cross training because of the similarity to running, ease of use and reduced impact. There are several brands, each with a slightly different design. You may want to check out a few health clubs to find a model that feels comfortable for you. Keep a towel handy because you will sweat more stationary than if you were outside running.
Two studies have evaluated training on ellipticals and found the improvement in the ability to use oxygen (VO2 Max) to be similar to running and stairstepper training. However, the effect on running performance (race times) has not been tested.
My wife has been using the Schwinn 430 for the past 6 months (she really works it hard five days per week). It is priced much less than most ellipticals and so far has been durable and reliable. We tried an Ironman before this unit and were very disapointed.